Sounds like swapping out the chain is worth a punt, my vague logic is that the chain length on this system is critical as it also determines the ‘b gap’ that’s no longer adjustable, so while I agree a shorter or longer chain wouldn’t normally cause an issue with just one gear on a cassette, a...
I wonder if there’s an anomaly on the sizing calculator for your bikes.
If you use the sizing matrix instead of the calculator it shows you should be cutting your chain to 120 links:
I fitted XX myself this week and the only observation I would add from your video is that your cage looks ridiculously extended forward in gear 1.
Mine is running great but my derailleur doesn’t extend forward anything near like that in gear 1.
Are you certain you’ve got your chain sized...