This was a hard one to track down, which I was able to repro in ASP .NET, ultimately I believe it was due to rewriting some async libraries to use async/await from manual ContinueWith() Task callbacks, which had a knock-on effect that resulted in an never returning async callback.
To minimize async compatibility issues like this in future I’ve ended up rewriting all async callbacks to use async/await and ConfigureAwait() where possible in this commit which resolved this issue.
This change is available from v5.9.3 that’s now available on MyGet.