6.1.5.2 I would never touch, but 6.1.5.9 being just one update away, I don't think that is too farfetched to use until a problem gets solved. Being that 6.1.5.9 was just worked on, according to the Changelog for missing Pluto videos, I wouldn't expect Pluto to work totally proper with it, though.
After doing test downloads with 6.1.6.0 and yt-dlp, I came up with the same results as you, Germania. Good Job!, once again. I think something got broken when they worked on the other Pluto problem in 6.1.5.9.
I am sending this endless "analyzing" problem for Pluto over to the Developer, right now.