Bugs when trying to continue developing MMF2 game in Fusion

I was making a game in MMF2 back in 2013 that I now want to continue development on it in Fusion to release on Steam. However there is a lot of bugs in the game now when running it in Fusion that was not in MMF2. The game is using box2d physics so I assume that is what is causing the most trouble.

Some strange things are happening, mainly if I run the game in Fusion runtime the game crashes at the point where I introduce a new enemy. If I build the game exe though and run it, it does not crash at that point (though it crashes later on at more random points).

Also this enemy is not destroyed when dropping bombs on him like he is supposed to be (and like the first enemy is, even though they have the exact same events). I am sure there are some other bugs also that I have not seen.

How can I fix these things though? Mainly the crash issues?

I was thinking of potentially going back to MMF2 for this game, but the problem there is that the Steam object is not supported in MMF2 I think?

Quote from CoffeeCup

Did MMF2 ever have box2d? I thought that was one of the big selling points of Fusion 2.5. I hope this isn't a common problem; I'm about to continue developing something I starting in MMF2 with Fusion 2.5. I'll keep an eye on this thread.

Yes it had the box2d object. (I am 99% sure I made it in MMF2 originally)

I am still getting these crashes and problems though. Is there any crashlog or something like this in Fusion so I know what causes the crash and strange behavior?

Also anyone know if the Steam object is supported in MMF2? I remember I released my first Steam game in 2015, but I don't remember if Fusion was around by then or if we where still using MMF2?

Another thing that is strange, but could it have to do with that the game was made in MMF2?

If I open my latest mfa file from 2013 the mfa file takes up about 16mb, but if I then save it to a new mfa file it only takes up 7mb?