You can check diff between two snapshots/subvolumes
sudo btrfs send --no-data -p {snapshot1} {snapshot2} | sudo btrfs receive --dump | grep 'update_extent\|unlink\|chmod\|chown' | cut -c 17- | cut -d ' ' -f 1 | sort | uniq
You have to set timeshift-path of snapshot in {snapshot}
It can tell you why.