I don’t have a clue, but it costs me nothing to guess.
During the brief time when the LH low beam is working, does it look the same as the RH? Like is it brighter, indicating it’s somehow getting a higher voltage (unlikely unless you incorrectly installed that nuclear plant under the seat), or does it flicker?
I’m putting my money on an intermittent joint. Filament-based bulbs draw a large inrush current on start, typically 10X their running current, which is a massive caffeine hit to the filament. Too much of that in a short time will kill them.