Ran into this as well when porting a new target and trying to get all constellations of test suites up and running. It seems to be an issue with how thread joins are handled during RtxThreadExit in RTX 5.5.0. Tracing the disassembly in the kernel makes me think there's some sort of optimisation bug in the precompiled libraries, since the idle thread is marked for running instead of the test thread (which was waiting on the join).