macOS is a certified unix, and after the postgresql people took a look at the situation they found out that it also loses data in that situation. So do OpenBSD and NetBSD.

Only FreeBSD would avoid data loss on fsync() failure. AFAIK that remains the case, the only major change I'm aware of is that OpenBSD's fsync is now "locked" into failure after the first failure, at least until you close the file (postgres also played silly bugger on that front).

The only certified UNIX you've listed is macOS, and it doesn't surprise me in the slightest that their frankenkernel doesn't fully comply with POSIX spec. Besides that, I don't know what to tell you. The whole business of "true UNIX" is pretty much thrown to the wayside these days anyways, the list of server-ready POSIX-compliant OSes is tiny, and their market share is even smaller.