Back in the 80's we used to modify 3spd transmissions to use in our dirt latemodels. Occasionally we would come across one that popped out of gear, our quick/dirty fix was to change the profile of the interlock detent notches in the side cover. I know nothing of the a855, but mis-indexed detent notches might be slightly side loading the forks, keeping the struts/springs from seating correctly.
I have cut large holes in the top of spare main cases, making them fixtures for easily checking the indexing of the interlock detent notches. I currently faceplate my own transmissions, which gives me the opportunity to change the placement of the dog rings and shorten slider travel distance. It's pretty much necessary to also modify the interlock detents for proper alignment to go along with the shorter slider travel, as you don't want to damage the forks due to un-necessary side loads.
Grant