So just to summarise: You get a magenta cast when contacting all the negs on a contact sheet of paper when presumably using the white light from the enlarger?
You get a perfect print using a test strip at f16 and 7 secs but the same magenta tone returns when you use the same over all exposure with F11 and 3.5 secs. When doing the test strip did any of the other exposure such as 2,4, 6 secs etc have the magenta cast or were they all free of the cast?
Everything else about your process was identical with both the magenta cast prints and the perfect prints?
If all of the above is correct then I don't get it either. Are you sure that it was exactly as I have summarised and is my summary correct?