I see the issue. It's unfortunate because this is a Darwin-specific fix, and I had to add it to mem_bsd.go since the two files were merged together since the last release. That's why this leaks into FreeBSD.
Also, the failure is kind of funny; MAP_FIXED does too good of a job using the arena hints it seems, but I'll investigate and try to figure out a fix ASAP.
Ohhh, no I see. MAP_FIXED is causing us to map over already mapped space without error. Ugh. This was expected, but I didn't think it would cause issues in tests. That kind of makes sense though; nothing else uses MAP_FIXED.
I put up a change which makes it so that MAP_FIXED is only used if GOOS == "darwin" (so FreeBSD gets fixed) and then disables the test on Darwin in race mode because the test is also likely broken there, it's just that the builder is failing earlier and masking it.