If there is a problem with the nut and it's not tight, consider this:
I reckon the reason why the problem with the front sprocket nut exists is as follows:
Minor play between the sprocket and shaft, due to shaft or sprocket being machined out of tolerance.
Nut tightened up on the sprocket.
Sprocket moves fractionally back and forward, and because the nut is tight to it, so does the nut.
Nut threads wear out as do the shaft threads.
Possible solution: Grind down the over-lap of the nut so it tightens up on the shaft and not on the sprocket.
In essence it should be flush with the sprocket but not tightening up against it.
Ideally make sure there's a layer of grease between nut / security washer / sprocket.