No overhang curling issues here, I had that as part of my calibration phase, and indeed it was all in the fan placement.
Just tried the same print again, this time with a all at 30mm/s as you suggested. I got the same layer shift. This time it was slightly further than before at about 15mm displacement. It happens later in the print with the lower speeds so far.
I have measured the stepper drivers with an IR probe at 55c at around the time of failure. I couldn’t find a listed thermal shutdown temperature for DRV8824’s, but I’ve seen it for other drivers at 160c. The steppers are running at around 40c.
I’m about to re-calibrate the vrefs, but I’m not so sure it’s speed related now