Good spot! I went back to the machine and gave it the old wiggle test and discovered the left Y motor pulley has a bit of play, the grub screw on the flat of the shaft was loose, not enough to let it spin but enough that there is backlash!
Disassembled the motor mount and reseated the grub screws with some blue loctite before I left for work this morning - I’ll run another test cut when I get home but I am hopeful that’s the issue - the backlash as it takes up the slack on the Y axis.