I know it sounds obvious, and you're a good shooter (we see that clearly!)
but every shooter is reminded to watch the thumb on support (left) hand
to make sure it isn't bumping slide stop during recoil. Just mentioning it .
Remove slide, put slide stop back in place, insert loaded magazine so
you can see if each cartridge is clear of the slide stop lug, or maybe
brushing up against it as the gun cycles. It doesn't take much
to lift it up and lock the slide.
Look at the lug on the slide stop pin to see if it is marked with copper. If it is, most likely the bullet is nudging the slide stop pin up, locking the slide back. You can carefully file a little off where the copper is showing try again and repeat if still locking back. Alternative is to order a new slide stop pin like a Wilson Combat Factory Plus. High quality inexpensive drop in part.
From looking at the video, I would say that your left thumb may be the problem. I know thumb position has caused problems for me locking the slide back when it should not and allowing the slide to close when it should stay open. I have modified my grip and have not had either problem in a long time. Everybody has hands that are a bit different, and a grip that works for one person may cause malfunctions with someone else.
This may not be your problem, but finding out whether it is or not is easy and very inexpensive.
edit: The fact that you can easily slingshot the next round into the chamber indicates to me that the magazine is not pushing up on the slide stop. If it were, you would probably need to depress the slide stop manually to close the slide.