Could you import them into a UE4 project outright(it could be a new one or one you have but a new one probably works best) then afterwards check if they are distorted or not then. If not, then the next move is to open that project with 5.1 and then check if they distort after that project got upgraded, if not, then you should be able to safely migrate them out.
It sounds a bit much, but this will also tell us if it’s just the asset itself.