Mysterious rear backup light bulb error

[WeekendDriver]

I was fiddling with the tail lights of my saloon B6 yesterday, specifically - I replaced them with aftermarket LED units. After installing them I got the backup light error for both left and right bulbs. And here's the funny part: these lights I installed are only semi-LED; they still have regular P21W bulbs for backup and turn lights. And not only the light bulbs are installed; they work and come on when I put the car in reverse.

So: the original tail light - no error, the new light - error, the new light with the backup light bulb from the original light - still an error. The lamps come on in all 3 cases. And I can absolutely clearly see that the backup light is wired directly from the socket, there's no electronics in the way or anything else that might obstruct the central electrics controller.

Ideas?..

[b1337]

Shitty bulbs

[WeekendDriver]

Tried two sets of bulbs, mind you, Philips and some noname. Both shine well. And yet, depending on the housing, I do or do not get an error.
???

Same for right and left housings. Doesn't seem very random, there certainly is a pattern here, I just fail to see it / understand the physics of this phenomenon.