Sorry I gave incorrect advice earlier, I misread your first post and didn't watch the video until after you posted that the water pump pulley wasn't installed.

In your video, I did notice that the two alternator belts were not parallel.
The front belt was actually in the air conditioner groove position on the main crank pulley.
If you didn't move the front belt back one groove on the main crank pulley, this would keep the alternator belts non-parallel and that could account for your recent symptoms.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022