Maybe you need to fasten it tighter around the legs? I've noticed with some nappies (similar style, Bedbugs and FFs etc) that what looks right when DD is lying down gapes horribly when she moves her legs, I often go back and hoik it a bit tighter.
Last week Iris had a Baby Beehind on which did the same thing - very wet baby, despite me doing it up as I normally would (it's a popper nappy). I think she was just sat funny, because it's fine again this week. FC it was just one of those things.
not got one of these so can't comment- but once with a new nappy with sam he stood up and it was hanging down a little and he pee'd and it just ran out the leg hole and wasn't absorbed at all ikswim so maybe something similar