We got a bunch of snow here in Boston so I've had to give the car a little bit more of the beans than usual just to move in the conditions. I noticed that if I exceed 2-3 000 RPM in reverse or try to clear a small mound of snow the gear will disengage and pop into neutral. I'm about to take it in for service but I'm hearing that this may be "normal" for the 5MT because there is no syncro for reverse. I tried doing a bunch of searches but nothing is conclusive so I figured I'd start something here.
The car does have a Kartboy Short Throw NB* clearances for gear engagement are fine. and I'm using motul 75W Gear 300 transmission fluid. NB* It has been mentioned that transmission fluid can affect the holding power of reverse.