We noticed some JoinableTask/Project lock hangs due to the JTF holding a lock (and switching to UI thread) is not added to the lock service collection, even those JTFs are created with the lock aware JTF factory.

It is caused by a special optimization logic in JTF library.
https://github.com/Microsoft/vs-threading/blob/022a5269613e7093c2ffb6c7a1074b8d416e268b/src/Microsoft.VisualStudio.Threading/JoinableTask.cs#L572-L575

The condition here to skip the event when synchronouslyBlockingMainThread is true is not correct. The current JTF is UI thread blocking task doesn't ensure that SwitchToUIThread won't be blocked due to several reasons:

1. the UI thread can run into a child JTF.Run on the top of the earlier task. The inner JTF.Run will not allow the task related to the outside JTF.Run to go to the UI thread by default.

2. the UI thread blocking JTF may have completed. It is very common a spin off task (either Task.Run, or Dataflow.Post can easily leak JTF, or it can be captured by many places capturing ExecutionContext, like a task continuation point, or cancellationToken registration) can retain the old JTF, which may have been completed. A completed UI thread blocking JTF doesn't provide any advantage for the later request to switch to the UI thread.

In both cases, the pending JTF will not be tracked by the lock service due to this issue, and leads product to run into a dead lock.