Nope, shouldn't be like that at all. Two cents;
- be careful bending the mag feed lips, it can create problems on top of problems you already have, leading to a high level of frustration.
- are the mag lips already bent/damaged? Kind of sounds like it. Compare to another mag...
- is the fifth round the first out of the mag? So, top round pops out, leaving four in the mag, then when empty the bolt is locked back?
I'd try another mag first, if possible. In the gun, and to compare. Again, careful if you're going to try bending the mag lips.
ETA: ah, upon rereading your post, seems the last round pops out, i.e.: bang bang, bang, bang (round pops out of mag), hold open engaged.
I wonder also if the mag body has become widened, enlarged, imagine cleaning out the mag with a broomstick.
K, turning this over to the experts, they'll be here soon.
ETA v2.0: not common, so far as I know. Norinco M14's are better known for crappy flash eliminators.