Repro:
RayTracerRepro.zip

1. Ensure you have a recent nightly build installed (https://github.com/dotnet/installer/blob/main/README.md#installers-and-binaries)... I have 6.0.100-preview.4.21179.9.
2. Unzip this project (an old parallel loops demo app).
3. dotnet watch
4. Click the parallel check box (it repros without it, but not as fast or consistently)
5. Click the Start button. The ball should start bouncing.
6. notepad Raytracer.cs
7. Edit L139 (return new Color(0, 0, 0);) to change one or more of the arguments to be a double value between 0 and 1 (e.g. return new Color(0, 0, .5);. Ctrl-S to save.
8. Repeat step 7, changing the values and saving, until this is printed out by dotnet watch:

watch : Hot reload of changes succeeded.
watch : Exited with error code -1073741819
watch : Waiting for a file to change before restarting dotnet...

The AV ends up coming from the patched method:

OS Thread Id: 0x5e54
Child SP IP Call Site
000000306157D1F8 00007FFE81620D04 Microsoft.ParallelComputingPlatform.ParallelExtensions.Samples.RayTracer.TraceRay(Microsoft.ParallelComputingPlatform.ParallelExtensions.Samples.Ray, Microsoft.ParallelComputingPlatform.ParallelExtensions.Samples.Scene, Int32)
000000306157FC00 00007ffe814a2d7d [DebuggerU2MCatchHandlerFrame: 000000306157fc00]

cc: @mikem8361, @tommcdon