I agree about the magenta shade in badly preserved film, but that I observed both in shadows and highlights. In my case the adverse effect was creeping here and there, not uniformly spread on the entire roll, strangely enough.

My notes say that a reddish tint on shadows could be due to a too elevated concentration of pre-bleach. Even if that is the case, I don't know how to relate it to your kit which uses "blix".

How do you control first development temperature? Maybe there is a problem with that. I wouldn't know how to explain, in that case, that the effect on the roll is not uniform, unless there is a separate problem with uniformity of treatment (e.g. horizontality of the tank during the treatment with a rotary processor). You should place the level on the tank itself when checking.