Adjust rear derailleur - strange behavior
From the video it does indeed look like the derailleur is being prevented from moving properly. You need to determine where the cause of that is. I think that most likely it's the cable or housing that is the problem.

1) Examine the shifter cable and housing, make sure there are no kinks, tight radii bends etc. Make sure the housing is fully inserted in any bosses, including shift levers and derailleur barrel adjuster. Make sure there are no obstructions to any sections of uncovered cable.

2) Remove rear wheel, shift gears at the levers and check the derailleur is going through its full range of motion and not sticking, especially on upshifts (cable loosening).

3) Disconnect the shifter cable from the derailleur, move the derailleur through it's range manually. Check there is no stickiness and the parallelogram spring is positively returning the derailleur to the high (outer) position.

4) Hold the shift cable at the derailleur end and put a little tension on it, run the shifters through all the gears. Check the cable is moving freely as you go through all the gears.

This should make it obvious if it is the derailleur itself that is sticking, or the cable, or possibly the shifter.