One thing you have to wonder is why NASA would allow this to be publicly released to the masses. Why didn't the astronauts stop when the lights went out? Why wouldn't they do another take? Why didn't the astronauts even comment on it at all?
Now, you know it isn't a floodlight going out for a few reasons. First, when it does "go out," the top of the tire on the rover is still fully lit, and appears to be lit in the direction of the supposed floodlight. Why was one floodlight so powerful to be able to light up the whole thing, yet the second isn't; and for some reason lights up only the wheel and no surrounding area? The explanation in the description as to why you can still see the astronauts' suits is that the light is coming from their helmet lights. The funny thing is, no Apollo spacesuit had lights on the helmets. The explanation for the wheel cover being visible is a "light patch," but then you have to wonder, why is no surrounding land lit up? Why is no other part of the rover lit up?
Your supposed "object" in the upper right corner is quite obviously just the mountainside. You can see it briefly just before the light goes out.
Going back through all the footage from the Lunar Surface journal, the phenomena happens quite often. It's very obviously just the camera's exposure settings being adjusted by mission control back on Earth. They must have accidentally set the exposure too low, and then corrected it when they noticed the error.
Unfortunately, there is no sound in the youtube video (odd, isn't it?), nor does the maker of the video give us any indication as to WHEN this happened in the mission, and during what spacewalk, so that doesn't help at all. I will make an effort to identify exactly when this occurred, but until then, I hope this answer satisfies you.
No, a floodlight did not go out. The camera's exposure was just adjusted incorrectly. It's a much more logical, and realistic answer, and when Occam's razor is applied, the better answer.
Edit: finally found it. occurs here:
https://history.nasa.gov/alsj/a16/a16.sta8.html#1473136 at about 2:16 into the video. No comments made on the dimming, but in that whole catalog of video, as I sat through it all, skimming through it, I saw many similar occurences. There was even one when the camera was pointing at the sun. It is only the exposure being changed.