Hello all. I’ve completed some serious work (Everything from stuff as simple as brakes all the way to engine rebuilds) on over 50 cars in my lifetime. I’m having an issue that has me absolutely stumped. I’ll be as descriptive as possible with my issue.

Background: A couple months ago, I bought a 2011 BMW X5 with a bad ZF 8HP45 transmission. Upon pulling the pan it was confirmed to be a mechanical failure (metal bits in the pan). I replaced the transmission from a donor (wrecked) 2011 X5. I removed the valve bodies from both the bad and donor transmissions. I then removed the mechatronics from the bad transmission and installed it on the new one’s valve body since they’re married to the DME.

Issue: From a stop, on all of the time on left hand turns, and sometimes right hand turns: the vehicle takes off fine, but as the turn starts has erratic shifts. It’s almost like the engine dies and then comes back to life with a jerk. It is not slipping (i.e. the engine RPMs do not climb without the vehicle accelerating).

Facts:
-The transmission was filled per the ISTA procedure.
-No check engine light, pending, or stored codes.
-Shifts in a straight line at all different applications of the throttle (quick starts; slow starts) are perfect.
-If I manually select the gears using the M/S option, the issue remains.
-With stability control off (DSC), the issue remains
-I completed the steering angle calibration using ISTA
-I changed the fluid in the transfer case and performed the calibration using ISTA
-I removed the front driveshaft and took it for a drive. Issue remained.

My thoughts:
-It sure feels like a classic case of a trans with low fluid (fluid sloshes away from the pickup on turns). But, I filled it per the procedure.
-Steering angle sensors can soft fail and cause weird transmission shifts. I would think, however, that with the DSC off it would take the steering wheel angle sensor out of the equation. I’ll probably still unplug it and see if the issue remains.
-Wheel speed sensors can also cause weird shifting, but they would likely through a trouble light.
-The TCM (mechatronics) could be bad, but again, I would think that a trouble code would be in.

Without a trouble code, I’m at a loss. Any thoughts from the community?