I was able to reproduce this reliably at the change 02a5502 listed in one of the failure instances, by repeating this 100 times:

gomote run user-danscales-openbsd-amd64-62-0 go/bin/go test sync -test.run TestMutexFairness -test.count=1 -test.timeout=15s

Using this same way to reproduce, I found that it started happening when Ian turned on the new timer code. It is fixed (not reproducible) in the current master, and by bisecting, I found it was fixed by Ian's commit

d80ab3e : runtime: wake netpoller when dropping P, don't sleep too long in sysmon

@ianlancetaylor does that make sense to you (seems reasonable to me based on the description of the change and the bugs that it fixed). If so, I think we can close this bug.

(Also, doesn't seem like it has been happening on the builder for the last week, as expected.)

Yes, I expect that was fixed by the timer change.