I have been getting them too and its been a bit annoying on pushes. Similar setup, using Tmax 400 (TMY older version) 1:100 rodinal semi stand for 1-1.5hrs. They are more noticeable on the edges of the film, and some have even gone into the image area.

I have guessed that the cause is surging of the developer through the holes during agitation, and I have reduced my initial agitation time, as well only doing 1-2 slow inversions at the halfway mark.