Hey Tom, hey Tom, hey all,
While thinking of your issue a bit, I think Tom is right with you first got to investigate what the possible causes are. I agree that EMI is only one thought that you could have. Another is a cable and connector problem. This is not related to spindle, but switching to the spindle may have been accompanied by movement/connection/disconnection of other cables.
The cables and connectors are a weak spot on these machines. The curly Z cable may be a spiral cable, but it is not made for motion like drag chain cables are and a break of this cable internally, first causing Z positioning errors, is reported more than once about one year after putting into service.
See also retrofitting strain relief on the Onefinity.
The Onefinity CNC manufacturer addresses various rails movement problems in this FAQ document:
The pattern could also have something to do with a loose Z coupler. Did you check this?