Thanks both, I've attached a patch which should hopefully fix the problem (and an image of what I get after applying the patch). Previously, it was assumed by proced-grammar-alist that the function proced-format-start would always return a string of length six which is not the case as shown above.
The patch fixes this by changing the justify attribute of 'Start' in proced-grammar-alist to 'left' instead of '6' (a fixed number means the attribute widths should always be this value - see the doc of proced-grammar-alist for more info).
I don't think JUSTIFY supports 'center' which I think would be nicer in this situation, perhaps this could be done in a future patch. AFAICS the only other use of a fixed value for justify is the 'nice' attribute, but I think this 3 char limit will always hold, at least for nix systems. Do you experience formatting issues for any column other than 'Start' Matthias?
I've also found another source of flakiness for the proced tests, but that's another issue (:
Thanks, Laurence
0001-Fix-Proced-Start-column-alignment-for-different-loca.patch
proced-start-formatting.png