I'm the creator of that video.
It seems this bug might not be new to iOS 26. I've experienced increased occurrence of typing errors ever since iOS 16 but always assumed it was due to my own clumsy typing, it was only recently that I decided to record my typing in slow motion to find out what was going on.
This bug isn't consistent, it doesn't always happen when I type the same phrase and it might even affect different phrases for different people, or not affect some people at all. Everyone types differently and has different devices with different settings, I suspect a specific combination of these is allowing the bug to occur but because there are too many variables involved it's difficult to pinpoint what exact combination of conditions can cause this.
I'd say that if you haven't noticed any unusually high incidence of typos while on iOS 18 then you won't notice it get any worse on iOS 26.