I can't replicate this on apfs at all, though I've also about 10x as much free space. Rather not write out 500GiB of data to try replicating.

    $ df -k /; dd if=<(yes) of=$HOME/bigfile bs=1024 count=$((35 * 1024000)); df -k /; rm ~/bigfile; df -k /
    Filesystem   1024-blocks      Used Available Capacity iused               ifree %iused  Mounted on
    /dev/disk1s1   976265452 360477544 612444948    38% 1399186 9223372036853376621    0%   /
    35840000+0 records in
    35840000+0 records out
    36700160000 bytes transferred in 1038.398720 secs (35343033 bytes/sec)
    Filesystem   1024-blocks      Used Available Capacity iused               ifree %iused  Mounted on
    /dev/disk1s1   976265452 396346588 576575904    41% 1399343 9223372036853376464    0%   /
    Filesystem   1024-blocks      Used Available Capacity iused               ifree %iused  Mounted on
    /dev/disk1s1   976265452 360497876 612424616    38% 1399342 9223372036853376465    0%   /