The "usual" on this is a weak magazine spring.
When the last round pops up out of the magazine like this, it's a weak spring not putting enough tension on the cartridge, and it can pop up out of the magazine.
Other "possible" causes:
An improper, or mis-installed spring. On military surplus guns, it's not uncommon to see the wrong parts installed by a previous owner.
The rifle is not properly assembled. In this one, the magazine may not be fitting into the bottom of the receiver properly. If there's a gap between the receiver and the magazine, the rounds may not stay in place or feed correctly.
First step is to check to be sure the rifle is assembled correctly.
Second, buy a new magazine spring made for THAT model of rifle.
New springs for most Mauser models are available from Gun Parts Corporation and Springfield Sporters.
