However the good news is that Apple is aware of the issue and in the recently released iOS 11.2 beta for developers, it appears that Apple has addressed it. For those unfamiliar with the bug, basically it had nothing to do with how the app calculates numbers, but rather with the animation used in the app that caused it to misread entries when the numbers and symbols were pressed too quickly.
For example rapidly typing in 1+2+3 should result in 6, but what some users were getting was 24, which is obviously wrong. In iOS 11.2, Apple has removed the animations from the calculator app which means that users will now be able to rapidly enter numbers and have it register correctly.
Of course this means that users will need to wait for iOS 11.2 to be released, which at this point in time is unclear when that might be, especially since iOS 11.1 has yet to be released either. In the meantime users can always turn to third-party calculator apps, or perhaps slow down their entries to prevent any mistakes.