There’s some confusion in your question, which I’ll try to untangle, but there’s also the strangeness that what you’re reporting doesn’t make sense in view of how things work, so there’s something non-obvious going on.
/var/snap/<snap>/ is not “USER_DATA”, it’s the parent directory of both
$SNAP_USER_DATA is typically
Snaps with data in any of these directories can be removed perfectly fine. The contents of those directories are backed up before the directories are removed (the
--purge option to
snap remove skips the backup).
So there’s something more going on: something you or your snap has done that the system can’t remove.
Can you tell us more?