I think you mounted the damper backwards, got the installation instructions right here to double check and it says it opens up with it facing the back.
According to the manual, you installed the vent panel backwards. Flap hinge supposed to be towards the front, so flap opens up and towards the front. ??? Probably doesn’t matter, but not per manual. Just sayin’
The cover on the left side is half vented and half solid, does this mean the microwave still does some venting back into the room? Why did they do it this way?