All 4 belts seem to be pretty similar in tension.
That was 5 or 6 passes at 0.3mm DOC at 5mm/s.
I’m not overly concerned about it.. my current aluminum milling needs are all through holes/paths, so I won’t even notice it on the finished product. But, if I can fix it, all the better.
It probably is rough spots on the rails.. I might take some high grit sandpaper over them and see if it makes any difference.