There used to be a bug in the msys2 runtime which made certain processes hang on exit in a non deterministic way.

So the parallel runs was disabled. I was looking into it but haven't been able to reproduce it at all in months now since either upgrading msys2 or Windows (to Windows 10).

We have a ticket with more information on it and what I found back then, but haven't been able to progress.

It might be that they've just fixed it in the mean time.