I'm also using the beta and would think the most likely is everything is working correctly, just some mistake or confusion, also Krusader showing wrong used space is likely a Krusader issue, I don't see if btrfs isn't freeing the space how Krusader could calculate that, please try this:
btrfs sub create /mnt/disk4/test
fallocate -l 50G /mnt/disk4/test/file
Wait a couple of secs and confirm used space changed by 50G, then:
btrfs sub snap -r /mnt/disk4/test /mnt/disk4/test/snap
rm /mnt/disk4/test/file
Now delete the snapshot and used space should go down by 50G after 30 seconds or so.
btrfs sub del /mnt/disk4/test/snap
After don't forget to delete test subvolume:
btrfs sub del /mnt/disk4/test