There's nothing like reading Kodak's instructions! In their publication about B&W films, they say that if magenta remains, the fixer needs to be replaced and that the film should be re-fixed. I did that with my magenta films and the magenta cleared out. The surprise is: Density increased! Both TMY2 and Acros had retained magenta, and here are their graphs before and after re-fixing:
Re-fixed curves are red. I could see why removing dye would decrease fog and thus increase densities by the same amount, but these don't do that. Both of these look like the result of additional development. Densities are still too low, but I don't understand why re-fixing increased them at all.
BTW, when measuring the red TMY2 curve, I used many frames of different exposures of the step-wedge, so there is no run across either row on the step-wedge. Furthermore, I measured Acros the same way, and there is no slope-transition in it. That makes me suspect that the two slopes in TMY2 are really there after all.