An example of a player being stuck inside of Elfheim castle's treasure room, on the Nintendo Switch version
The stuck in a room glitch is a game-breaking glitch that occurs only in the first Final Fantasy Pixel Remaster. It is known to happen on the Switch, Steam, Android, and iOS versions. It is unknown if it occurs on PlayStation.
The trigger conditions are not completely known. After acquiring the Mystic Key, it may be possible that quick-saving within certain rooms (such as the north-eastern treasure room in Elfheim castle) will render it impossible to leave. When the player tries to exit through the door, the game will not register them as being outside of the room, causing them to be permanently stuck inside as they cannot move any further than one tile out.
There is currently no way to recover from this glitch other than by reverting to an earlier save. If no earlier save exists, the player is forced to restart the game.