diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js
index d5d30bfc87c67..2c61e1b95b988 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js
@@ -29,10 +29,6 @@ const unionType = typeBuilderApi => {
     name: buildTypeName(type.name),
     types,
     resolveType: node => {
-      if (node.type) {
-        return buildTypeName(node.type)
-      }
-
       if (node.__typename) {
         return buildTypeName(node.__typename)
       }
@@ -104,7 +100,7 @@ const interfaceType = typeBuilderApi => {
   } else {
     // otherwise this is a regular interface type so we need to resolve the type name
     typeDef.resolveType = node =>
-      node && node.__typename ? buildTypeName(node.__typename) : null
+      node?.__typename ? buildTypeName(node.__typename) : null
   }
 
   // @todo add this as a plugin option
diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/helpers.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/helpers.js
index d08ab8ce82697..0afa669a0dd34 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/helpers.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/helpers.js
@@ -24,6 +24,10 @@ export const buildTypeName = name => {
     name = `FilterType`
   }
 
+  if (name.startsWith(prefix)) {
+    return name
+  }
+
   return prefix + name
 }
 
diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/index.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/index.js
index e72877387a96f..fa6dd48b747f2 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/index.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/index.js
@@ -5,6 +5,8 @@ import { fieldOfTypeWasFetched } from "./helpers"
 import buildType from "./build-types"
 import { getGatsbyNodeTypeNames } from "../source-nodes/fetch-nodes/fetch-nodes"
 import { typeIsExcluded } from "~/steps/ingest-remote-schema/is-excluded"
+import { formatLogMessage } from "../../utils/format-log-message"
+import { CODES } from "../../utils/report"
 
 /**
  * createSchemaCustomization
@@ -96,7 +98,13 @@ const createSchemaCustomization = async api => {
   try {
     await customizeSchema(api)
   } catch (e) {
-    api.reporter.panic(e)
+    api.reporter.panic({
+      id: CODES.SourcePluginCodeError,
+      error: e,
+      context: {
+        sourceMessage: formatLogMessage(e.message),
+      },
+    })
   }
 }
 
diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/default-resolver.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/default-resolver.js
index de217f48e02f9..1c3c8e55ad8ec 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/default-resolver.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/default-resolver.js
@@ -1,6 +1,7 @@
 import { findTypeName } from "~/steps/create-schema-customization/helpers"
 
 import { buildGatsbyNodeObjectResolver } from "~/steps/create-schema-customization/transform-fields/transform-object"
+import { buildTypeName } from "../helpers"
 
 export const buildDefaultResolver = transformerApi => (source, _, context) => {
   const { fieldName, field, gatsbyNodeTypes } = transformerApi
@@ -45,6 +46,12 @@ export const buildDefaultResolver = transformerApi => (source, _, context) => {
     finalFieldValue = aliasedField2
   }
 
+  if (finalFieldValue?.__typename) {
+    // in Gatsby V3 this property is used to determine the type of an interface field
+    // instead of the resolveType fn. This means we need to prefix it so that gql doesn't throw errors about missing types.
+    finalFieldValue.__typename = buildTypeName(finalFieldValue.__typename)
+  }
+
   const isANodeConnection =
     // if this field has just an id and typename
     finalFieldValue?.id &&
diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-object.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-object.js
index 6c23ef581df32..2037b153963ea 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-object.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-object.js
@@ -55,6 +55,12 @@ export const buildGatsbyNodeObjectResolver = ({ field, fieldName }) => async (
 
   const queryInfo = getQueryInfoByTypeName(field.type.name)
 
+  if (!queryInfo) {
+    // if we don't have query info for a type
+    // it probably means this type is excluded in plugin options
+    return null
+  }
+
   const isLazyMediaItem =
     queryInfo.typeInfo.nodesTypeName === `MediaItem` &&
     queryInfo.settings.lazyNodes
diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-union.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-union.js
index 052e57ced694a..b79f568344e66 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-union.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/transform-fields/transform-union.js
@@ -16,12 +16,10 @@ export const transformUnion = ({ field, fieldName }) => {
 
         if (gatsbyNode) {
           return gatsbyNode
-        } else {
-          return null
         }
       }
 
-      return resolvedField
+      return resolvedField ?? null
     },
   }
 }
diff --git a/packages/gatsby-source-wordpress/src/steps/ingest-remote-schema/build-queries-from-introspection/recursively-transform-fields.js b/packages/gatsby-source-wordpress/src/steps/ingest-remote-schema/build-queries-from-introspection/recursively-transform-fields.js
index 0ee6a7bca018f..8245f8d3e947a 100644
--- a/packages/gatsby-source-wordpress/src/steps/ingest-remote-schema/build-queries-from-introspection/recursively-transform-fields.js
+++ b/packages/gatsby-source-wordpress/src/steps/ingest-remote-schema/build-queries-from-introspection/recursively-transform-fields.js
@@ -303,10 +303,10 @@ export function transformField({
   const isAGatsbyNode =
     // if this is a gatsby node type
     gatsbyNodesInfo.typeNames.includes(typeName) ||
-    // or this type has a possible type which is a gatsby node type
+    // or all possible types on this type are Gatsby node types
     typeMap
       .get(typeName)
-      ?.possibleTypes?.find(possibleType =>
+      ?.possibleTypes?.every(possibleType =>
         gatsbyNodesInfo.typeNames.includes(possibleType.name)
       )
 
diff --git a/packages/gatsby-source-wordpress/src/steps/source-nodes/create-nodes/create-nodes.js b/packages/gatsby-source-wordpress/src/steps/source-nodes/create-nodes/create-nodes.js
index 262056ebd449d..e99f4bd2ba751 100644
--- a/packages/gatsby-source-wordpress/src/steps/source-nodes/create-nodes/create-nodes.js
+++ b/packages/gatsby-source-wordpress/src/steps/source-nodes/create-nodes/create-nodes.js
@@ -48,13 +48,16 @@ export const createNodeWithSideEffects = ({
     })
   }
 
+  const builtTypename = buildTypeName(node.__typename)
+
   let remoteNode = {
     ...node,
+    __typename: builtTypename,
     id: node.id,
     parent: null,
     internal: {
       contentDigest: createContentDigest(node),
-      type: type || buildTypeName(node.type),
+      type: type || builtTypename,
     },
   }