1. It's likely that your insulation material isn't stable enough. You can take some insulation material and run a thermal stability test.
2. Oxidation or annealing isn't working properly.
3. It's likely water damage. If it's corroded, it won't change color if placed in clean water. When testing insulation resistance on wires, they're usually placed in water. Sometimes, even after being left in water for two months, they still don't change color.
