Odd resistor failure

This may seem off-topic at first, but I realize that folks here do SMPSU repair a lot. My thought was that it might be applicable, since they're basically the same thing--the ballast runs the lamps with a 30-40KHz AC voltage. I've been going over a small stack of failed T8 bulb solid-state fluorescent lamp ballasts from about the mid 90's. They all have the same failure and it suprised me. There's a 2W 0.33 ohm carbon film resistor used as a current sensing element. In all failure cases, the resistor has failed open, with no signs of burning, but rather the outer paint flaking off. To me, this is a puzzle. At 120v, the current through an 0.33 ohm resistance in series with a 64W load is about half an amp. I2R gives less than a tenth of a watt power dissipation across the resistor. (There's also a 2A fuse in series with the whole circuit). I replaced the failed resistors with 5W composition ones of the same value, and they seem to work okay. But the original failure has left me scratching my head. Does anyone have an insight on this type of failure? --Chuck