It appears logic the #warning is addressing matches non-fscrypt directories behavior. This logic has been here since 2009:
https://github.com/ceph/ceph/commit/85d893c5f5a3d85331e09ff89587330392c016fb
per stat(2)
st_blocks
This field indicates the number of blocks allocated to the file, in 512-byte units. (This may be
smaller than st_size/512 when the file has holes.)
on ext4 filesystem:
build$ truncate -s 16M file1
build$ stat file1
File: file1
Size: 16777216 Blocks: 0 IO Block: 4096 regular file
Device: 8,17 Inode: 84200845 Links: 1
..
This isn't observed in ceph as it is based off of just file size: https://github.com/ceph/ceph/blob/1b53353dbdcb1a34745495fb7f27945380adf995/src/client/Client.cc#L8635
non-fscrypt dir in ceph:
mycephfs$ truncate -s 16M file1
mycephfs$ stat file1
File: file1
Size: 16777216 Blocks: 32768 IO Block: 4194304 regular file
Device: 0,45 Inode: 1099511667168 Links: 1
..
fscrypt dir in ceph:
fscrypt_test_unlocked_ffsb$ truncate -s 16M file1
fscrypt_test_unlocked_ffsb$ stat file1
File: file1
Size: 16777216 Blocks: 32768 IO Block: 4194304 regular file
Device: 0,45 Inode: 1099511667186 Links: 1
..