Sadly, all the field cases I’ve owned going back to the 5/5S did this. They’re inexpensive and great cases otherwise, so I’ve just accepted it as the way it is with Magpul field cases.
I have found a way to minimize the issue somewhat. My phone rides in a vehicle mount for most of the day. Choosing a mount that 1) doesn’t squeeze the sides and 2) has the bottom feet near the corners instead of the middle area around the port seems to help.
Probably still replace every 3-4 months though, as I have had a phone partially come out during a drop when I let the deformation get too far along.