ColSebastianMoran
( IRL Richard Karash )
Completely different and wild guess... Mould grew on the emulsion. Washed out a shallow "crater" during processing, creating a darker spot on the print. Wild guess...
Definitely bubbles - I have seen the exact same thing before. You said that you made the D76 and used it right away. Heating the developer and mixing the powder also dissolves air in the water. The bubbles form as the water 'outgasses'. Try this, fill a glass with hot water, and then let it cool. Look at the surface of the glass - see the bubbles? Same thing. That is why it is a good idea to let freshly mixed developer sit for an hour or so to get rid of the dissolved air as much as is possible.
OK, first post after lurking for a while 🙂
I just scanned a roll of Rollei Retro 100 with the same "white spot" issue. In fact, I was just going to make a post about it when I saw this.. Good to know what they are, the negs look almost like there are little "pits" in the emulsion and of course the while spots are very time consuming to fix in post.
Developed it in Rodinal 1+50, and thought I paid real good attention to smacking the tank on the floor (my bathroom is small, OK 😀 ) after every agitation. I'm always afraid of breaking it (early 1980s vintage Paterson tank) but perhaps I should try smacking it harder.