Interesting.
This is always going to be challenging as it is easy for a ham-fisted person to keep tightening a truss rod and strip the thread, then how do you prove it's a manufacturing fault not user error?
As you are clearly not a clueless beginner and tehre's a known fault with these guitars, I have a theory: the truss rods haven't been threaded for a long enough distance so you simply run out of adjustment. In running past the end of the thread, the nut strips.
It's likely the upper parts of the thread are still fine. If you still had the neck, I would recommend seeing if you could still wind the nut right off, get a new nut and use it with a spacer 6-10mm long on the still intact end of the thread.