The roll of warm-white LEDs I used for the first sewing machine lights has evidently aged out:
They’ve been wrapped on their original roll, tucked in an antistatic bag, for the last five years, so it’s not as if they’ve been constantly abused.
All the cool-white LEDs on an adjacent roll in the same bag still work perfectly, so you’re looking at inherent vice.
I harvested the three longest functional sections and dumped the remainder in the electronics recycling box.
Admittedly, I haven’t looked at the RGB LED strips in a while, either.