2021 Silverado with 57k just started doing the same thing. Cheap parts I assume. When I pull down to signal it flashes the brights. Likely just cheap plastic internals in the lever that have worn down and it's now able to make the connection with lights switch and turns them on. If I'm super careful I can keep it from clicking over and not flashing, but it's become a complete nuisance.
Still a bit of a work-in-progress... been having a hell of a fun time learning how to weld and this trailer is going to end up with way too many fun little things added to it