I had a quite similiar experience recently, I used it in a project that was corrupted and when opening it GameMaker showed the following message
"This project is of an older format and will need to be converted. Proceed?"
Then I opened the project and one of main objects was gone just like that, it can be because when my computer shut down it was the object I was working on but it is what it is, I wish he updated the tool though..
Com certeza foi a primeira ideia de muitos, mas logo se conclui que isso é inviável porque as pessoas testando o jogo com certeza não vão ter mais uma pessoa para testar ele e isso vai diminuir drasticamente as reviews do seu jogo, se quer muito mesmo colocar co-op, dê a opção de jogar o jogo começo ao fim sem ele também !
The game has potential for real! but this version of it is just lacking a lot of things that make or break a game, I really see this being a thing if you put a little bit more effort and time, overall a cute, short and fun game!
A arte do jogo é linda, mas senti que pecou na gameplay, porque o jogo não possui replay value e é extremamente curto, mas para sua primeira Game Jam está ótimo, continue se dedicando que você vai longe!
I know it's hard man but are you really good at this, see how the others artists do their assets pack and make one yourself, I would say that's better to make a new asset than to keep updating this one, I believe on you man, I really do, believe in the process.