Alrighty.
After dicking with #Klipper for two days, there's something wrong with the kinematics.
Being a Hybrid #CoreXY, my kinematics have a lot less people using them than a run of the mill CoreXY.
Hybrid CoreXY means that two steppers have to work together to make what might be a simple, lateral Y- toolhead move.
I am not an expert at #Klipper by any means, but I've noticed a couple instances when this seems to be true.
There's a small bump at homing (which could also be Klipper backing off, in case it was already flush with the sensor) but there is also an example near 0,0 when clicking the button to move Y, only moves the X stepper when it should move both.
This shouldn't happen. It needs to operate BOTH the X and Y steppers together to make that move. Even if it cannot move one (e.g. it's at the limit), it should then abort the move on the OTHER stepper. It should never move ONLY one when both are required.
So; After two days, it's back to #Marlin.