What happens when a power metal film resistor is exposed to higher power?
These resistors, according to the data sheet, have "Defined fusing characteristics". This means that the components will act like a fuse at some level of overload and go to open circuit. 

There are graphs, starting on page 12 of the linked data sheet, that show the time to interruption as a function of overload power. You did not specify the ohms value of the resistor that that you are using in your circuit but the graphs for the PR02 part series would show you the range of time to interruption (going open circuit). So lets say for example if you were using a 1 ohm PR02 resistor the fusing time can range from about 0.35 seconds up to 20 seconds when the overload is at 55W.



At less than 55W overload the vendor is not willing to specify what the fusing characteristics would be. The most likely scenario is that the resistor would get smoking hot and could very well allow damage to other parts of the circuit. The resistor is supposed to be built in a manner that it will not sustain flaming (at least the coating is flame retardant) but the generated heat could very well cause adjacent non-flame retardant materials to catch fire.