It might be the magazine catch. Try holding the magazine up when firing that last round and see if that solves the problem.
Ive seen them do exactly what your saying. It's catching the last round as the mag is sitting lower than it should causing the round to feed at a bad angle.
Seems even more likely after second thought .
Never know for sure when the thing isn't in your hands to examine it. Even then can be a challenge.
Say maybe put two rounds in it and do what James said first.