hello! that is a bit of a bug with the save system, unfortunately, which i'm working on fixing. hopefully it didn't impact your experience with the game too much.
if you want to replay the game at this point, you'll have to delete the save files which should be located in the AppData\Roaming\Godot\app_userdata\UNDERTALE flashgames folder. this is my first game, so some bugs are to be expected, but i'm happy you enjoyed it!