Seconding what Starrynights said... I have a large collection of MH, and my dolls have experienced a universal yellowing of white hair and vinyl.
I remember quite a few years back, when G1 MH was still on shelves, there was already discussion amongst collectors about the werecat twins’ hair starting to turn. It doesn’t seem to be caused by storage methods, sunlight, temperatures, or any other environmental factors, as my dolls have yellowed evenly no matter where they’ve been kept. In my experience, sunlight may accelerate it, but nothing at all seems to prevent it.
Unfortunately, it’s pretty much untreatable. Since synthetic hair isn’t porous like real hair, the color is unlikely to respond to bleaching or peroxide. It’s one of the sad but inevitable truths in collecting that nothing stays new and perfect forever... especially not plastic. If it’s any consolation, there aren’t going to be any other collectors with better Abbeys, unless they’ve been rerooted—they’ll all go yellow in the end.