What could cause a Caterpillar 621E scraper with a reman transmission to not shift properly until put into 6th gear?

[USVeteran]

Looking for some help troubleshooting a shifting problem on my Caterpillar 621E scraper. The machine is a 6AB806, and it has a reman transmission (0R4025), but it will not shift correctly unless I move it into 6th gear first. We've already done some testing, and I'm trying to figure out if we're missing something electrical, hydraulic, or inside the transmission control system itself.

 

[Gryks Machines]

That's an interesting one. If it only starts shifting properly after hitting 6th gear, I'd be curious whether the issue is in the transmission control valve, a sticking solenoid, or maybe a calibration problem from the reman unit. Have you checked if all the clutch pressures are within spec during the shift sequence? Also wondering if the wiring harness or shift position sensor could be sending an incorrect signal until 6th gear is selected. Did the problem start immediately after the reman transmission was installed, or did it develop later?