just like the vintage Barbies get greasy because the plasticizers are breaking down so are the FR getting brittle from their chemical make up.
Shock BonBon turned yellow and I never had her stored or exposed to harsh light and always AC and still she turned.
Kyori was well cared for and I went to adjust her head and her neck cracked in my hands so her beautiful robe stayed on her body and became a manikin and I got the head a made to move Barbie body
because there weren't any bodies available and I didn't want to spend a hundred dollars.
That's why I won't invest hundreds of dollars obtaining grails.
It's also why I debox my dolls and play with them

Also this time frame has seen the AD Gene dolls and some Tonners become discolored and brittle. So it's coming from the plasticizers degrading that were used at the time in the factories.

A lot of glue's are also starting to break down and some of my expensive barely worn human shoes have fallen apart sitting in my dark AC'd closet open on the shelfs.

Keeping the dolls boxed up and away from air circulation is not good for them and seems to accelerate the problem.

The other thing I have also noticed with my BJD's is the elastic is stretching out on dolls that are sitting with no pressure on their limbs or bodies and still the elastic is degrading.

You even notice brittleness in rubber bands they become hard and just crumble or melt.

It's in the formulations of the plastics.