Just developed a roll of HP5+ 120 and was shocked to see the mottle when I scanned the film. Naturally I thought it was a fault in my process. So I was quite relieved to find this thread and know it is a documented problem with others. Posted my problem to Ilford support as was suggested here.
I wish I hadn't thrown out the backing paper. My guess is that it is a problem with the ink used on the side of the paper with the printing since that side is in contact with the emulsion. Lots of changes in the ink industry trying to limit volatile gases and toxicity. One scan has the mottle in a bar across frame. Bet it could be traced to backing paper!
I guess from now on it's load, shoot and process in short order. No more leaving a loaded camera or back sitting around.