Just to make sure there are no other issues, try just using the coupling nut. No spring, no second nut. See if that keep everything smooth. It can be easy to overtighten the anti backlash. I might just sell the kits with a longer coupler nut and skip the whole anti backlash. I’m not sure its is needed or is even strong enough. Kind of hard to test really.
Clean the coupler nut before you clamp it in for a solid clamp.
Just to cover all the bases. The most recent one I put together was having z issues, turns out it was kind of lopsided. The threaded rod was visibly crooked. So i loosened the whole middle assembly gave it a wiggle to seat everything and tightened it back up. Works perfect now. So check to see if the rod is equal distance from the bolt heads.
Maybe its time to design a new 2 piece middle.