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).