Quad Shot Breve Mocha

Like many people, I frequent a barista on my way to my (sometime stressful) marketing/graphics director job.
Again, like others I know, I don't always drink it before it gets cold. So naturally I warm it up in the microwave for about 1.50 minutes.

In the last few weeks I have noticed a change in cup materials that causes the base of the cup to begin to burn in the microwave and smells like I just lit our paper bin on fire. Photos attached. This also happens with whatever Starbucks is using too.

Very weird! A side effect of using even 10% post consumer recycled paper?