I was an avid calculator spotlight user; however, ever since Catalina CPU spikes have been insanely high, I have stopped using it. And most of the other users who noticed this started posting about it this year, so I am sure that there was some change that was rolled back that caused spotlight to stop recognizing when a calculation was being invoked (but I could be wrong).
I think it’s a bug because a) 100%+ cpu usage is ridiculous even for normal spotlight searches past the initial spotlight indexing operation and b) this definitely worked without an issue or insane cpu usage before Catalina (checked on old MBP on Sierra).
The option to use spotlight as a calculator is part of its feature set. And the fact that it still does recognize (after a lot of cpu usage) that I wanted to make a calculation does indicate that the calculator functionality is still there. It just seems to be triggering some huge operation that requires even more cpu usage than a normal spotlight search.