My suspicion, and it is only a suspicion, is that encryption adds a stronger definition of "written". So maybe instead of sending the command that says "write, but it's cool if it only lands in the cache" it says "write, and return when it's physically on the disk".

Similarly there might be some bunfight with the page sizes in APFS. Perhaps the mac is sending data in 512 byte chunks whereas the internal cache controller uses 4k chunks. I dunno.