Hmmm - I was printing from the pdf that opened in Firefox, as I always did with the 30-up.
I just downloaded the label, opened with Adobe Reader and it printed correctly. So it seems Firefox is the culprit! Still at a loss for why, but at least I can get printing and prepping now!
edit - maybe Firefox was the problem with the 30-up alignment?? The top margin was very small and Amazon’s scanners did not like that.