Okay, so if I understood right, the bug wasn’t from the save files themselves but from the config.rpgsave file that RPG Maker MV creates (that’s the one that stores keybinds and options). If carry an old config over, it can mess things up and cause bugs.
The fix is just to delete config.rpgsave in the save folder, the game will recreate it with the default settings, and everything should work fine again.
So, if anyone else runs into this, try removing the config first before touching the saves, right?
Good to know it finally got resolved! 🎉