Thought KDX's had front rim locks.
Anyways, if you are tightening up the valve stem nut down to the rim, that is the problem! Back the nut out so that it tightens up against the valve cap. The stem can still move that way, but will not get pulled through the hole. Do the same on the rear even though you have a rim lock.
Back in the old days when some bikes had no rim locks on the rear, we would tap small holes into the side of the rim and put small sheet metal screws into the tire bead, four per side. It worked, but there never seemed to be a need to do it on the fronts. I think you can buy aftermarket rim locks, but you would have to drill a big hole in your rim.
Good luck solving the problem.