It's been a while since I had to replace a bulb, but IIRC, even if only one of the 3 bulbs in either tail light clusters fails, the light comes on even if it appears that the light still works. So, maybe one of the three bulbs is burnt out.
I had a bad ground on my rear light which was causing the rear left bulb out indication. I followed a DIY from here to add a new ground wire and the problem has never recurred since (over a year now). I fixed both sides.
(Only time I had a bulb out shown for the front, it really was a dead bulb.)
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