Are you sure it wasn't just some conductive surface contamination, of the component or PCB? Not far between the end metalizations on 0603.
That's kind of the opposite problem to one I've found several times now in cheap switchmode supplies. Some of them use a 1M resistor for a trickle start-up of the switcher on the mains side. So there's an approximately 1/4W metal film resistor with near full rectified mains voltage across it. They eventually go open circuit, so the supply never starts. Even though it would run fine if it did start. And because it's a 1M resistor, and the surrounding circuit has more leakage than that, with the resistor in circuit you can't tell it's open.
I think it must be a kind of electro-migration of the metalization on the resistor.
Replace it with two 470K R in series, and they work.