find exhibits the same behavior for me as @tavianator.

I've tracked it to fstype.c:250
if (entry->me_dev == statp->st_dev)

It appears the predicate is never true for me in my test setup and `best` is never assigned.

~/$ stat -c "%d %n" /mnt/btrfs-test /mnt/btrfs-test/asdfsdaf/subvol/
1048804 /mnt/btrfs-test
1048805 /mnt/btrfs-test/asdfsdaf/subvol/

~/$ df --output=fstype,target /mnt/btrfs-test /mnt/btrfs-test/asdfsdaf/subvol/
Type  Mounted on
btrfs /mnt/btrfs-test
-     /mnt/btrfs-test/asdfsdaf/subvol

~/$ find /mnt/btrfs-test -printf '%F %p\n'
unknown /mnt/btrfs-test
unknown /mnt/btrfs-test/thing
unknown /mnt/btrfs-test/asdfsdaf
unknown /mnt/btrfs-test/asdfsdaf/asdf
unknown /mnt/btrfs-test/asdfsdaf/subvol

Breakpoint 7, file_system_type_uncached (fstype_known=0x5555555a32b0 <fstype_known>, path=0x7fffffffde80 <incomplete sequence \344>, statp=0x7fffffffde80) at fstype.c:245
245     in fstype.c
(gdb) p *entry
$14 = {me_devname = 0x5555555af940 "/dev/loop98p1", me_mountdir = 0x5555555af960 "/mnt/btrfs-test", me_mntroot = 0x5555555af980 "/", me_type = 0x5555555af9a0 "btrfs",
  me_dev = 1048793, me_dummy = 0, me_remote = 0, me_type_malloced = 1, me_next = 0x0}
(gdb) p *statp
$15 = {st_dev = 1048804, st_ino = 260, st_nlink = 1, st_mode = 33204, st_uid = 1000, st_gid = 1000, __pad0 = 0, st_rdev = 0, st_size = 0, st_blksize = 4096, st_blocks = 0,
  st_atim = {tv_sec = 1617846997, tv_nsec = 768596091}, st_mtim = {tv_sec = 1617846997, tv_nsec = 768596091}, st_ctim = {tv_sec = 1617846997, tv_nsec = 768596091},
  __glibc_reserved = {0, 0, 0}}
(gdb) c
Continuing.
unknown

Breakpoint 7, file_system_type_uncached (fstype_known=0x5555555a32b0 <fstype_known>, path=0x7fffffffde80 <incomplete sequence \345>, statp=0x7fffffffde80) at fstype.c:245
245     in fstype.c
(gdb) p *entry
$16 = {me_devname = 0x5555555af940 "/dev/loop98p1", me_mountdir = 0x5555555af960 "/mnt/btrfs-test", me_mntroot = 0x5555555af980 "/", me_type = 0x5555555af9a0 "btrfs",
  me_dev = 1048793, me_dummy = 0, me_remote = 0, me_type_malloced = 1, me_next = 0x0}
(gdb) p *statp
$17 = {st_dev = 1048805, st_ino = 256, st_nlink = 1, st_mode = 16893, st_uid = 1000, st_gid = 1000, __pad0 = 0, st_rdev = 0, st_size = 0, st_blksize = 4096, st_blocks = 0,
  st_atim = {tv_sec = 1617847022, tv_nsec = 444810726}, st_mtim = {tv_sec = 1617847018, tv_nsec = 344774790}, st_ctim = {tv_sec = 1617847018, tv_nsec = 344774790},
  __glibc_reserved = {0, 0, 0}}

As @berny points out this is likely because of the discussion in https://savannah.gnu.org/bugs/?50326 ; But I'd like to understand why df can make this distinction where find can't.

Ah okay, you're actually parsing the device number out of /proc/self/mountinfo?  When I implemented -fstype for bfs, I didn't bother getting it from there, as it's not returned by getmntent().  I just stat() the mount point itself, which obviously works.

On a btrfs file system, find -fstype btrfs doesn't find any files, and find -printf '%p %F\n' prints

. unknown
./.git unknown
./.git/refs unknown
...