I came upon this Reddit thread and thought, “Impossible. This can’t be true.”
But yes, it is. Try this yourself. When you hit the plus sign for the second time, the calculator app goes into some odd state. Not sure if this is intentional, or a bug, but either way, this is not what you’d expect.
The good news? PCalc gets it right.
UPDATE: Looks like this is an animation lag issue and goes back at least through iOS 9, possibly earlier. Also, Apple employee #8 weighed in about filing a radar.