Search of rec.games.pinball for triple thick and saran wrap.
Any fix like this is risky and not guaranteed, but what is guaranteed is that left like it is, your piece will continue to deteriorate.
I have a Black Hole backglass that had a couple of sizable pieces loose.. I did the triple thick, stuck the saran wrap to it, pressing the pieces into alignment and weighted down/flattened.. removed the saran wrap when dry, peeled right off. Mine worked well, but I could see it going either way. You really have to get some of the clear up under the loose paint for it to work. Good luck.