Must be a hairline filiment fracture ...both lows should be on the same relay ? (please check fuse box, underhood relays) and if you had a ground problem your light would come on but be dimmer, so I'm betting it's the bulb.
I have had headlight bulbs that worked most of the time...and then would not work until they were "thumped"...and then one day they eventually fail, once the bulb was removed you could see the filiment breakage.
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