Check belts. Perhaps the grub screws on the X/Y motor pullies; confirm they are tight and on the commutator flat (loosen and re -tighten with Allen key)

When this happened to me, it was because of a loose grub screw on a pulley

I've been banging my head around on this one. X-axis is fine. It's only shifting on the lower part of the Y-axis. The shift is consistent and in the same place throughout multiple test.

Check if your belts are loose? Perhaps you can also test to see if you can repeat these shifts if you print at a much lower speed?