Extension prevents markdown preview

[garthk]

PlantUML extension 2.13.1

No other extensions enabled

PlantUML server running on 8080 in Docker container from image plantuml/plantuml-server:jetty

VS Code version details:

Version: 1.40.0
Commit: 86405ea23e3937316009fc27c9361deee66ffbf5
Date: 2019-11-06T17:09:34.601Z
Electron: 6.1.2
Chrome: 76.0.3809.146
Node.js: 12.4.0
V8: 7.6.303.31-electron.0
OS: Darwin x64 19.0.0

Markdown document:

# Demo

```plantuml
@startuml
testdot
@enduml
```

settings.json:

{
    "[markdown]": {},
    "plantuml.render": "PlantUMLServer",
    "plantuml.server": "http://localhost:8080"
}

If I delete the fenced code block, VS code's Markdown preview works. If I put it back, the preview stops updating.

[qjebbs]

pls update