All I can say is that I have a set of old filters which are labelled MGII filter set and they reflect exactly what the OP has found so even the older set of filters still had an increasing magenta look until grade 3 /3.5 after which the grades 4,4.5 and 5 look lighter and less magenta.

It might be that my higher grade filters have faded more but as they would have been used less, this seems unlikely.

I cannot give any reason why, unlike the colour head dual filtration, the filters don't get progressively more magenta but they don't