And tight on the RD cable is really *tight* before attaching the fixing bolt...odds are backing off even a faction from, "as hard as you can pull" will get these symptoms. Could also be as a result of excess housing (bends too big) or too little housing (too small)....OP didn't say if he was messing around or installing from scratch.

No matter what, there is excess cable that is slack above what you're plucking in the video, OP...and the shifter's index ratchet is taking that slack up instead of pulling on the RD.