I restarted brand new (with no additional patching) with 15.2 and the game broke on the shovel.
Just loaded 15.3 and the same things that broke from 15.0->15.1 broke again with the 15.2->15.3. They really should work on those variables if they want to properly advertise keeping your saves. As it stands right now the retention of saves isn't even alpha quality since each hotfix regresses previous errors.
Okita's keycode popping up immediately after loading the save and her state machine being reset BACK to the start, but you can't progress it since the key code is already in your inventory. The last time I looked at the RenPy they didn't have a single linear stack of flags for each person, instead of juggling multiple in separate variable names. Having a single stack would make their logic checking much easier, and less likely to encounter these same issues over and over.