UPDATE: The problem was that I was not completing the equation. When I wrote out the full equation, Math Input Panel did, in fact, correctly recognize the equal sign.

I wanted to end up with this equation:

So, I started writing:

When I wrote the equal sign, I got a capital sigma:

So, I tried to use the correction feature.

The equal sign pops up as an option, but, when selected, the resulting equation is nonsense: