Nappy folding in over?

My son's nappies have been doing this the past weekish. It's not affecting the function but I can't imagine its comfortable for my son. It's an all in one birth to potty sized nappy, velcro fastening. Not sure why it's doing this? I've checked the fit multiple times and it's fitting fine, not too tight or too loose. Any insights? He is mobile and always running around/climbing so not sure if that's affecting it somehow.

Edit - This is the other brand we use but these nappies have also been folding over. My son is sucking his belly in, in the photo so it looks loose but it isn't when he relaxes. To see fit of nappy when first put on.