I think button touches aren't triggered until animations finish so if you do 1 + 2 + 3 real fast, the second "+" won't register.

Hmmm it could be but who knows? Almost there entire eco-system is closed off so ¯\_(ツ)_/¯ I guess we’re at the mercy of an update so we can get back to calculating things with at least a few order of magnitudes of accuracy...

It's not hard to know what it is. Watch the screen. You can see the animation.

The animation doesn’t mean anything. You should be able to have many animations happening in parallel and their completions should not block keyboard events from being handled...

keyboard events are fine — with a Bluetooth keyboard, you get the right answer no matter how fast you type.

The animation shouldn’t prevent the + button from being pressed again, but it does. That’s the bug.

Or try typing it slower and you'll see that the result is correct

You type it the wrong way. Do not worry, Cook will explain how to type correctly.