This is a known issue. A very thin metal strip in the LED assembly cracks -- and it's sealed in the assembly. As of Nov 2019, GM is finally replacing the tail light assembly under warranty but only for 6 yrs or 72k miles. If you're outside of this range you're screwed and these are basically guaranteed to crack eventually. Incredibly poor design for any vehicle, let alone one designed to go bumping down rougher roads.

GM does not consider tail lights safety critical. In my case, I have a "salvage" title due to a minor collision when still new on the dealer lot (how they got it designated as totaled is beyond me) -- GM will cover safety critical items on salvage vehicles, but they denied me on this. Who needs brake lights anyway? They're just a convenience.

You can find videos online on how to melt or cut the plastic away where this crack occurs (two places in each tail light), solder jumpers, and then reseal the assembly. Serious pain in the butt though -- then again, so is shelling out for a $500 assembly (x2 eventually).