A bit mysterious, but seems as files will now have nanosecond accuracy since 1901 (in the 64bit unsigned int), the quota timers are still 32bit and have been downgraded from 1 second to 4 second accuracy to cover the full bigtime forward time range required (for 1970 unix epoch).

Seems to be described in this patch comment: https://patchwork.kernel.org/project/xfs/patch/157784114490....