strange... you say both sides were working at one point when you "bench" tested the left light?.. and the problem did not move when you changed the ballasts, the bulbs?... the LCM has ability to "shunt" faulty circuit to avoid further issues and bypass the light... on the other hand, if the conditions are right, the light should come back...

do you have the ability to read the error codes from the LCM? it will list the error codes and their description if you use the BMW software to pull the codes... you might find the explanation there, like check driver's xenon not-plausible, error occurred 5 times, or 20 times or whatever...

did your left fog light come on too? there is a fail-safe protocol, when the fog lights come on if the main lights are not working...