In our case this is most likely caused by a bug in the garbage collector. When the service falls in to the frozen state, all the threads are deadlocked by the GC.

This is likely to be caused by one or more bugs in the .NET 9 runtime. There were various GC hang reports since the release of .NET 9 (1, 2 etc.), and the dotnet/runtime team has made several fixes accordingly. Our service was targeting an early version of .NET 9 runtime (9.0.102), after retargeting to the latest version (9.0.300), the issue did not appear again in our recent load test which spans 20+ hours.