> it's registering a U, highlighting U, and inputting J

The voiceover is deceptive (unintentionally?)...

They touch the [u] which shows the popover U but you can see them slide their thumb down off the [u] key onto the [j] key.

I guessed that was the issue, repeated it on my phone (SE) and only then looked at video and it's obvious when you see him do it in slo-mo. Edit: I have most prediction turned off (I mostly find slyde typing to be fastest, and I hate automiscorrect on uncommon words).

iPhones are very very sensitive to tap-slides which causes many UI gremlins (a variety of terrible side effects that you can't avoid if you're designing a UI).

Over time, most people seem to intuitively learn not to slide when tapping.

I'm unsure how many designers/developers even notice the effects of slide since they have learnt to avoid sliding? When I watched beginners on iPhones you see them get frustrated by things not tapping and other subtle effects (HTML event interactions, scrollable areas, buttons, inputs).

Same thing can happen on Android. One menu button repeatably failed if I used my left hand - took me a while to work out the issue (and a bit of work to increase the tappable area so a bit of slide was accepted and worked better for neophyte users).