I saw this same phenomenon in some J&C Pro 100 in 120 size that I had stored past its expiration date. I believe the film was rebranded Shanghai GP3 100. Some rolls from the same batch that had been exposed and developed before it expired also showed this, after doing some further checking, but was much less noticeable and I didn't see it at first. I only saw it after I had trouble with the last couple of rolls and went back to look harder at the earlier ones.
I also noticed this problem recently with a batch of 10 rolls of Ultrafine Plus 100 in size 120 that I bought several months ago. As a control, I exposed and developed a roll of Plus-x that was cold stored and expired a couple of years ago. No mottling, so I don't think it was my process. I think the Chinese emulsions in 120 are susceptible to this, especially past their expiration date even if cold stored. The Ultrafine film had no expiration date, so I have no way of knowing how old it is.
Edit: I just noticed you mentioned the negatives were Delta 100. Is the film expired, by any chance?