Car is a 5.3/4L80e. I had it running on someone else's ECM with an HPTuners custom OS. No idea what that tune looked like, but the transmission worked like it should.

I swapped the ECM to a fresh one so I could start working on the tune. I loaded up a 12212156 OS, segment swapped in the 4L80e Express Van transmission calibration and diagnostics, then did the COS3 upgrade. From there I did my best to put together a base tune, and tried to do some driving.

My transmission is commanding D4 shift pattern regardless of my shifter position. I know this can happen when P1810 is set, but I have no codes. (Well, I had P0452 because I don't have the EVAP in place, but that shouldn't change the shift pattern, right?)

Changes to the transmission cal:
WOT shift rpm's raised to 5800
Torque Reduction Tables zero'd out
Abuse Mode disabled

I can command any gear with the scan tool, just not with the shifter.

Any ideas on what could be causing this issue? Are there other conditions where the ECM would command a D4 shift pattern without setting a code?

Thanks in advance.