This is what I see, it's a bit long winded. TLDR, New fork and gears.
I see that while changing up (lever up) it is pulling the drum too far, then snapping back into the detent position.
Can you pull the lever up hard and hold it, then watch if the drum moves at all to let the big detent wheel set between the pins as you let it relax. If anything the detent wheel should be pulling it the last little bit into position with the lever up all the way.
It seems yours is having to pull it backwards to seat properly after traveling too far.
It's not happening changing down.
If it's ever been dropped on it's side while in neutral, and the gear lever has hit the ground and been forced up, and it could only go into gear partially because the gearbox was stationary, (like you are finding in the video, the dogs are hitting end to end instead of slipping in between. ) The gear wont be able to move far enough in, but the shift fork is still being forced by the gear drum, that is being forced by the lever. (Can also help shear off the top of the point in the selector drum groove.)
CB's are heavy. That may have bent the travel stop that prevents the lever moving past the ideal stopping position.
That above can bend the shift fork a little, so when it is in gear it's dogs are engaged slightly less than before, (which isn't much to start with.) It's the beginning of the end, every time it jumps out it takes a little more off the sharp edges of the dogs, until it does it all the time. Straighten or replace the fork and check the condition of the point in the drum groove.
Couple that with holding the lever up after changing into second, It's rotating the drum too far, (the selector fork has effectively gone over the top of the hill, in the groove in the drum, and started on it's way out of gear already, pulling the rounded top of the dogs into contact with each other and ramping it back out of gear against the fork.
False neutral between 2-3. drum was too far around, If it kicks out of gear back down towards the real neutral the drum wasn't pulled around far enough.
I'm a tight***, so I welded and reshaped the "top of the hill" in the drum groove and straightened the fork, actually made it engage further in than than the original. Ground the tops off the dogs, (not the sides) to get clearance between the gears with the new long reach fork, that now can reach the good straight edges of the dogs. Never jumped out of gear before I dropped it, hasn't jumped out since.
The thread above yours when I was looking contained this article which is related to what I'm trying to say about the lever travel.
http://honda-cb750-s.456789.n3.nabble.com/file/n4054341/MotorcyclistJuly1981ShifterArticle.pdfI think you will be splitting it, and replacing forks and maybe gears.
