16C is somewhat lower than I use with Rodinal - I try and stick to 20C (68F). I wonder if the low temperature is causing some weirdness?

My Rodinal combo for Acros is 1+100 for 18 minutes. Five inversions (in 15s) at the start of each minute for the first three minutes, then one inversion every three minutes thereafter. I always get excellent results with this combination.

Also, are you 100% sure this is not dust in your scanner? Have you checked with a scan of a different film immediately after?