Why am I only getting cold air from the ac vents on either side of the steering

What is your question?

I:m getting warm air out of the other vents. I'm also getting an occasional hissing sound from the dash/vents.

What makes this problem better or worse?

It's worse when the outside temperature is hotter

How long has your 2018 Mercedes-Benz E400 had this problem?

it's periodic