PCB Heater design issues
Thermal stability is not your problem. Either you are running your traces hotter than you think, or your substrate is not FR4. Your temperatures are high enough that the epoxy is failing and the board is delaminating. 

Since you have a temperature sensor attached to the traces, I'll assume your temperature measurement is accurate. So I'll focus on the substrate. Unfortunately, "FR-4" says nothing about the actual materials used in making a PCB. It refers only to flammability - "FR" stands for "Flame Resistance", after all. Worse, FR4 has become a generic term, like Kleenex, and there is no guarantee that any substrate labeled FR4 actually conforms to the standard. This site, for instance, lists continuous operating temperature for FR4 as 285 F (140C) minimum, so a real FR4 substrate ought to survive your situation.

I'd suggest finding another PCB supplier.