Prune juice left in cup?

During a birthday party, my 4 year old grandson hid some prune juice (in a glass) in the laundry room closet. He stacked the same size plastic cup inside the glass that had about 3" of prune juice in it.

When I found it a few weeks ago, the plastic cup had somehow sealed tight to the glass and the prune juice was the consistency of very thick tar. No mold, it just sealed tight and I m interested in what it turned into?