With my particular MKIII, both my MecGar 15 rounders will occasionally lock the slide back with one round left in the mag. 13-rounders don’t do that, so… I mainly use those. But maybe my particular gun has a janky slide stop or something.
It also kinda feels like the rounds “ka-chunk” off the feed ramp a little more with the 15s vs. 13s. But maybe that’s my imagination.