The whites on some on a number of my Bandai valks that I've had for a long time have either faded or flat out yellowed. When the white fades, it's only really only noticeable next to the painted parts and/or parts that used a slightly different plastic (or placed next to a brand new revival) but it's there.
Yamato valks I've had even longer have not really had issues with yellowing at all. Bandai seems to use a cheaper plastic, at least on these older toys. Only time will tell if they improved it on the revivals.
Check another box on the "Bandai cuts corners" list.