I think you'll find the issue is with the magazine. Because there is no detent or oring holding the pellets in place the magazine lets the pellet slide out into the breach area . I have the same issue with both the magazines I have . I've leaned to just live with it and only cock the gun when horizontal.
I you look at the back of mag there is 3 holes, increase the the spring preload and it will put more side force on the pellets and help it keep it in place during the load cycle , most of the mags are sent out on the softest set .
99% sure it is a magazine issue....Check the little brass guide pin that rides in the slot in the MRod magazine.... If that is pushed in too far during manufacturing, it drags, preventing the wheel in the magazine from rotating properly.... This can also happen if you have the center screw too tight.... From my experience, it is seldom insufficient spring tension, although that is often used as a "patch" to cover up the other problems.... Bob
the prob size is small and it will let mag rotate enough to drop a pellet out of top side of oval hole. My mag is like a FX mag. load one from front and rest from back.