what are you trying to do? A stiffness of 0 to start with will just let the object collapse completely on collision (particularly with a Structural of 5). Bringing the stiffness up over 80 frames is just going to make it want to retain that flattened shape even more. I’m guessing you want to flatten them but then return to round but I can’t be sure without more info. (if it is what you want you would need to use pressure (or an Inherit effector)).
Why it collapses when playing the timeline like it does I can’t tell, somethings wrong with the file I’d guess, when I make a fresh trial it works like I would expect.