Ah!mike wrote: ↑Sun Jan 07, 2018 4:50 amA bit of follow-up here.
After some further digging, it appears that the problem on APFS is not in F_PREALLOCATE itself, but in a consecutive ftruncate() syscall.
This doesn't change the whole picture, just a detail.
The Internet already has some WTF posts regarding this behavior.
Thanks a mil Mike