Better than POSIX?

All that is actually needed is for the metadata updates to be kept in sync with the data updates. Then the write-new-file-rename-over-old-file trick will reliably leave you with the new or the old file again. Getting data/metadata out of sync seems to me to be a bad idea even without the possibility of crashing.

It may not be required by POSIX, but it is a reasonable way for a filesystem to behave.