"...only happens on the last round..." No upward pressure from the following round.
Dunno if this will help, but it can't hurt. I'm thinking one or more of the follower parts are worn or broken. It's the same rifle as an 1889 Mauser when it comes to hunting for parts. Gunparts wants $12.65 for a Follower Arm w/ Spring.
http://stevespages.com/ipb-mauser-model1891.html