This corner block did not weave to square.
Then I tried using heavier pattern warp yarns to force the issue. In the photo above I switched my pattern weft from 10/2 mercerized to 8/2 unmercerized cotton. The problem persisted. Changing the sett from 30 e.p.i. to 36 or 40 e.p.i. would also be worth trying but then the towels would be narrower. That idea was quickly dismissed.
Another strategy would be to throw more shots for each treading block. I admit to being a purist on this idea -- with the right yarns, setts and beat the design should square itself. Once you manipulate the draft in this way other areas of the design tend to go out of whack. So I discarded this idea almost as soon as it came to mind.