It does seem like a pretty straightforward calculation, but for some reason when typing fast, it just doesn’t seem to register. Apparently this is due to an animation lag where if you enter numbers too quickly, the input will be ignored, which means that this bug is more to do with the animation used in iOS 11 rather than the app itself.
In a post on Reddit, user cplr writes, “Any iOS developers will see what is wrong here: the bug is that the animation that lights up the button is blocking touch events until the animation completes. This is the default behavior for animations, but to make an app feel responsive it’s best to find a way around that (it can be a one line fix, but sometimes it is complicated).”
Apple seems to be aware of the bug as Apple employee Chris Espinosa tweeted that more than 70 people have filed a Radar report regarding the issue. Whether or not it will be addressed in the next iOS 11 update remains to be seen, but for now users can opt to use a third-party calculator app or simply type slower.