I just had this exact same issue on the BL, surprised I didn’t see this thread when searching.
I was able to solve it by deleting the Maslow.yaml file on the machine, uploading a fresh one, and calibrating again. The problem came back on BR after the machine crashed, so I tried it again but updated my calibration numbers in the file before uploading and it fixed the issue again.
Check my thread for a video and details: Belts become loose on any axis movement - #30 by kyleschoen
Hope this works for you!