When I sync a folder using selective sync, not always, but sometimes, .bts files hang around after the folder is completely synced. However, if I unsync an image that should be linked to that .bts file, it creates a new .btsi file that I can use to sync back.
It's a little bit of a mess.
On Windows, there's something similar, but if I unsync the file individually (not a whole folder) and sync it back, the extra .bts file goes away.