FP4+/Rodinal marks on negative

Hi all. I am having an interesting problem with FP4+ and Rodinal 1:75. I am developing 4x5 sheets in SS hangars in yankee tanks, following Kodak's agitation recommendations. First off, I only see the problem on FP4 and Rodinal combination. I have never seen it with any other film or developer combination, including Rodinal on other films.

Now, the problem. On the base layer of the film, I am seeing circular marks on the film edges that correspond to the holes in the SS hangars. The emulsion side of the film is fine, no streaking, no agitation marks, no density changes, nothing. The circles (and often a faint outline of the top of the hangar and clip) only appear on the base side. At first I thought it looked like water scum marks or something, but they don't rub off. They are very faint, but visible, especially when looking along the plane of the film.

Any idea what these could be? Could the Rodinal have some sort of solvent action on the base layer of FP4 to cause these marks? They definitely only appear the longer the film is in the developer. Agitation does not seem to make a difference, as I have tried everything from vigorous agitation to complete stand/no agitation.

The good news is they don't appear in the print. However, they are really bugging me, as I don't like marks on my negative :)

Thanks for the help!