90hz-100hz ? - Waterfall

Hello folks,
howdy
I need some help to understand my waterfall.

My room is 25ft x 16ft and 8ft ceiling.

My mix position is facing the 16ft wall.My monitor is about 1-2ft away from the wall and my mix position is about 5ft away from the wall.

I have bass trap at my room corner and other trapping at the reflection point.

Here is my waterfall graph at my mix position.


I'm trying to find the answer why I have a big drop at 90hz-100hz.

For fun a test my room with my mic at the center of the room and now the drop at 90hz-100hz disappear



Someone have an explication?