Ian Grant
Subscriber
You are most probably seeing micro-reticulation due to variation in temperature. See this informative post by @Ian Grant who advises keeping temperature variation within +/- 1C throughout the process.
John Davies perhaps one of the best landscape (now urban) photographers in the world teaches in Liverppol. Henoticed that students all using the same films and developers had very differing results when it came to grain, some years ago he noted that the most uncontrolled factor was temperature control, and the poorer the students controls the more graininess.
Rodinal contains free Hydroxide and that softens emulsions some more than others, Acros was one film with issues but I think it used some Whale gelatin which is different to Bovine gelatin.
Ian