First of all, there is no "macrovision" - this is a company name (was
) and conversely encompasses about 3 major variants and some subvariations. There are other copy protections as well. One box does not solve all of them, not even a full pro TBC. One need a full TBC and a signal processor.
Anyway, back to your problem - and you should have done this BEFORE asking here - is this (d)effect seen on TV when playing the cassette? Can you confirm the signal leaving your "demacrovision" box has (or has not) this (d)effect? Can you insert a color proc or any other processor in order to fade the signal (if level too high it may be clipped or the DSP would react to it)?
If your observations are correct and complete, it's not "macrovision".