Oh. My. Gosh. It happened again. It's a MyBB bug and I will trace it down as soon as possible!
Tac
22 Mar 2017
Edited
I have just began experiencing this bug, currently unable to save any templates. Even with the plugin uninstalled.
Edit: The bug hadn't occurred before installing Thunderboard, but it could be a coincidence...
ThunderBoard has currently an issue which prevents saving templates unless you save them twice (or use TB's rebuild cache functionality). It's not related to FASTyle.
I have introduced some breakpoints in my dev version of FASTyle so whenever the bug will occur, I will catch it and see what's wrong.
Tac
22 Mar 2017
^ Fixed after rebuilding cache, thanks!
Still can't trace it down.
Edit: the issue is solved starting from 2.0.