does anybody know the reason why my truck does this?

I've put 2 sets of rpm arms on, a Tmaxx and Emaxx. I fitted them like you did - I used a drum sander on dremel until the arms were free to move under their own weight. Both sets have no back and forth play in the bulks .

If you have taken too much off one arm, get some washers and shim out the play. I would push arm forward and shim play out play in that direction. I think tyhat will come closest to matching the camber of the other arm. Ultimately, if its bad enough to be the cause of all this, need to replace that arm.

Sorry the trx jerk was a waste of time. I've had some good luck and some poor - ity depends on who you connet with and what kind of day he's been having, it can be hit or miss, some are incredibly helpful, knowledgeable, and patient. Maybe it was his day off??