So... Mid Side controls in channel tools are a little odd and probably little understood.
Basically, the mid control brings polarity inverted copies of both channels up in the opposite channels.
The result is not quite the same as increasing the mono. Instead, as you reduce mid, you raise polarity inverted left in the right and the right in the left. The summing nulls some of what is shared in both channels.
This is often used to create a sense of space while making each side more audible in it's opposite. When you sum to mono it cancels exactly.
https://www.youtube.com/watch?v=pmTb8XqawP8 Have you watched this one?