Something is either assembled wrong, or bent/bad. I have never seen the tail gears strip. the 54t gear in the front is what normally strips first and the front bevel gear being second, but only in a crash/hard tail touch.
When you rebuild, check mesh. There is a washer on the center spacer that can be moved to either side of the tail shaft bevel gear to adjust mesh on the mushroom. It's stripping, not melting, so I think your mesh is too loose. But hard to tell without seeing it in person.
Also be sure the mushroom gear is fully seated against the bearings. I've seen that push out when the TT is inserted and that will wreck the gears fast.
