What version of GNU make are you using? I've been using make 4.3 since its release in 2020. No changes, no prereleases. I'm afraid the problem, whatever it is, is not that simple :(.
What troubles me most is that there's no obvious way to debug any test failure involving parallelism, since they go away with serial execution. Any ideas about how to determine what is going wrong in the parallel make? Any way to make parallel failures more reproducible? Right now, all I know is "some random test(s) fail(s)". Not helpful. The test logs show the set -x execution of the test, so the actual command that fails can be seen. I have keep_testdirs=yes, of course, but then running the command by hand in the shell after the failure often does not reproduce the problem. Something else is going on, but my imagination about what that might be has failed so far :(. Argh. --thanks, karl.