You seem to have run into a problem similar to others when a btrfs file system becomes nearly full. It does not readily release the space, and there are some btrfs commands which may assist in recovering the unused space.
I do not use btrfs on a regular basis so am not familiar enough with the appropriate commands to make suggestions but others here should be able to provide the needed help.
These threads provide some info but I have no first hand experience and those are only 2 of many that may assist.