I had the same problem, the foam backing on the headliner material was breaking down due to heat & humidity (Florida car) only noticeable in the front corner just as you describe. Only solution was to have the headliner replaced. When I removed the headliner board, the material separated from the foam very easily. Had I let it go longer, I'm sure I would have come out to find the headliner dropped entirely. I chose to redo the headliner in a lighter/whiter shade but with the same weave as factory. I had all the hard panels, including the hatch trim covered. It came out really well, I also had the useless sunvisors deleted and the rear hatch pull down handle smoothed out and covered.