Earlier quoted context omitted.
Maybe I'm misunderstanding the problem being described, but I just tried this and it is impossible for me to type so fast in the iphone calculator app that it misses inputs. The animation completes in the background but does not block anything.
If I type "3 + 6 + 9" as fast as I can, then hit "=", I only get 18 about half the time. Sometimes I get 12, sometimes 15, occasionally something else. This is with a iPhone 11 Pro on iOS 14.4.2. It's ridiculous. I always hear the same number of clicks, though.
Somehow got the "6" key is stuck lit up, and despite having definitely registered a tap, it wasn't included in the addition.