The switchback guide in the exit assembly is staying open when duplexing causing the paper to exit to the duplex tray rather than the finisher. Single sided works fine both face up and face down. I can turn on and off shift solenoid with sim, then will duplex fine for a few copies. I have traded out the switchback assy, wire harness for that assy, eng board, main board, duplex board, duplex tray assy.

I've seen the feedshift gate stick on a pad which acts like a cushion for the gate. After a while the pad begins to degrade causing it to become sticky.

Manually open the gate with some pressure to determine if it's sticking. Lately I've been removing the pad to avoid future sticking gates.

Also may want to chech the white plastic arm attached to the feedshift solenoid to make sure it's intact.

I've been replacing that pad with the fuzzy/loop side of self adhesive velcro. It damps the noise but never gets sticky. On one machine I also replaced that solenoid.

This may also be pertinent:
Misfeeds (Jam J81) leaving accordianned, image paper face down at fuser exit, with front leading edge hooked into the feedshift gate fingers (1M prints). Upper right plastic guide on the duplex tray is broken at the clip, leaving no gap between the upper & lower guide at the front. Paper is deformed passing through duplex, then snags on the feedshift gate while exiting the machine. Replace right guide (2FB19060), two springs (302FB19180), two stoppers (2FB07330), Film (302FB19560), and C-clip (2BC06910).