"Once, cups were lined with wax. Today, paper cups are 95% paper and 5% polyethylene plastic lining."

I would think this is industry standard but I could be wrong.

Still, no matter the cup it may have some kind of chemical release agent coating to slide off the machines.

System Three logo paper mixing cups are on the shelf at my local epoxy dealer, so I assume they must be OK. Too expensive to use very much, for me.

Some years ago, I was using the flat bottomed clear plastic 'drink' glasses for mixing epoxy. I never had a problem with bonding or epoxy failure, but one day I noticed the mix was turning cloudy as I stirred it. Shortly afterward, it got more difficult to find those clear 'glasses', so it didn't matter. Both the 'red' and 'white' thin 'glasses' I can get locally have the indentations in the bottom mentioned earlier. They also dissolve in some solvents; I had the bottom fall out of a couple before I stopped using them.
Eat more yogurt! :-)