Running synaptic or Update Manager will call apt-btrfs-snapshot in the background. What I do (if I do not want to take a snapshot) is:
then reinstall it when I need it or want to take a snapshot.
sudo apt-get remove apt-btrfs-snapshot
I guess btrfs scavenges from the swapfile at will. How is why is beyond me because I do know that I have not been able to successfully do a btrfs install with a swapfile partition. Even on an all btrfs hdd, during initial setup, it would not install properly , reporting errors etc..