looks like the tooth on the mag catch has only marginal engagement with the step on the pmag. steel mag catch, polymer mag, with minimal engagement will lead to deformation and failure. with deeper engage of the mag catch tooth, the load is spread out over a larger area, would last longer. Lancer mags have metal up top? harder surface, so they're not as likely to deform with short tooth engagement? take a look into the mag well, see how much that mag catch tooth is hanging out.
JMHO, that's all.