The thing is the sag that I saw on mine tended to follow a pattern similar to yours. I can’t explain the exact forces or why cuts in certain directions make it happen like that, it just looks very similar to what I was experiencing before going on my leveling crusade. I have a fairly long forum post about this from over a year ago if you have time for some “light reading”, haha!!!
The set screws I’m talking about are the ones on the pulley for your Z axis stepper motor. If those are loose you’ll get all sorts of inconsistencies in Z travel, which could also cause this. I think these are now installed before the kit is shipped to you. It’s definitely worth loosening them, apply blue loctite (242/243) and reinstalling. Also worth checking that all of your V wheels are adjusted properly. Basic mechanical checklist stuff. Make sure when the machine is on, that if you grab the router you can’t move anything easily, steppers should be locked. Shake it a little, see if you feel anything moving. Really thinking you have something mechanically wrong here, either flex or something loose.
Dan