I suspect oily vinyl, maybe.
Not sure, but they are bright yellow spots. Look on dolls by sandy. That is how mine was. It will not go away from painting. BUT you can put macpheresons skin varnish over and it will mask it.
I had this with a Birdie by Laura Lee Eagles kit that my parents bought me for Christmas 2018.
It randomly appeared on the belly plate and I was not happy, there were also random oil patches that just wouldn’t go away even after I had thoroughly cleaned the kit - neither did the yellow marks which upset me and I had to sell the kit as a boo boo kit in the end