That's why Magpul PMags have a plastic clip on the bottom that can be put on the top when the mag is loaded, to keep the top cartridge off the lips, like they would be in the gun with the bolt forward.
Magpul says this isn't true. The cover is nothing more than to keep dust and dirt out. It puts no pressure on the cartridges.
For years we were assured by "experts" that aluminum AR magazines would spread the lips if left loaded, now disproved.
Now they're telling people that plastic mags will spread the lips if left loaded.
Top companies like Magpul say that too is not correct.