Comment # 8 on bug 931571 from Arvin Schnell

Btrfs deletes snapshot asynchronously, so after the delete command (both
btrfs and snapper) you still have to wait to get the disk space back (or use
btrfs commands that wait for the cleanup to have finished). That might
explain why the disk space appeared after the balance command.

If really a balance command is needed it looks like a kernel problem to me.