Add a code to reproduce the problem

Related
dotnet/runtime#53663
mono/libgdiplus#708

.gitattributes

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+# Auto detect text files and perform LF normalization
+*        text=auto

.gitignore

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+bin
+obj

Program.cs

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+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Drawing;
+using System.Drawing.Drawing2D;
+using System.Drawing.Imaging;
+using System.IO;
+using System.Linq;
+using System.Threading;
+
+namespace TestImages
+{
+    static class Program
+    {
+        static byte[] _jpegBytes;
+        static EventWaitHandle _evt = new AutoResetEvent(false);
+        static bool _running = true;
+
+        static void Main(string[] args)
+        {
+            int imagesOnCollage = int.Parse(args.FirstOrDefault() ?? "10");
+            int aCollageDrawingPeriodMs = int.Parse(args.Skip(1).FirstOrDefault() ?? "100");
+
+            //ThreadPool.SetMinThreads(100, 100);
+
+            Console.CancelKeyPress += (s, e) => 
+            {
+                _running = false;
+                e.Cancel = true;
+            };
+
+            _jpegBytes = File.ReadAllBytes("image.jpg");
+
+            while (_running)
+            {
+                var sw = Stopwatch.StartNew();
+                DrawCollageImages(imagesOnCollage);
+                int spent = (int)sw.Elapsed.TotalMilliseconds;
+                int left = aCollageDrawingPeriodMs - spent;
+                if (left < 0) left = 0;
+                Thread.Sleep(left);
+            }
+
+            _evt.Dispose();
+        }
+
+        static void DrawCollageImages(int count)
+        {
+            ThreadPool.QueueUserWorkItem(_ =>
+                {
+                    Console.WriteLine("thread id {0,4}, threads count {1,3}",
+                        Thread.CurrentThread.ManagedThreadId, ThreadPool.ThreadCount);
+                    using (var collage = new Bitmap(1000, 1000))
+                    {
+                        DrawCollageImagesImpl(collage, count);
+                    }
+                    _evt.Set();
+                    Thread.Sleep(20000); // prevent this thread id reuse
+                }
+            );
+            _evt.WaitOne();
+        }
+
+        static void DrawCollageImagesImpl(Image collage, int count)
+        {
+            using (var g = Graphics.FromImage(collage))
+            {
+                g.DrawCollageImages(count);
+            }
+        }
+
+        static Rectangle _rect = new Rectangle(0, 0, 500, 500);
+
+        static void DrawCollageImages(this Graphics g, int count)
+        {
+            for (int i=0; i<count; i++)
+            {
+                g.DrawCollageImage(_rect);
+            }
+        }
+
+        static void DrawCollageImage(this Graphics g, Rectangle r)
+        {
+            using var ms = new MemoryStream(_jpegBytes);
+            using var img = Image.FromStream(ms);
+            g.DrawImage(img, r.X, r.Y, r.Width, r.Height);
+        }
+    }
+}

SystemDrawingMultithreadingMemoryLeak.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="System.Drawing.Common" Version="5.0.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Update="*.jpg">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
+</Project>