Looking at the picture I don't see any vents except the built in ones in the front of the exo-terra.
What is the 3D background made of. if it is a hydroscopic material it could be soaking up all the moisture.
and hasn't gotten saturated yet.
also with he rise in ambient temp it is harder to keep humidity up also.
I also see that you are using a aquarium thermometer. That will take a bit to register quick changes
in air temp. I would get one that is made for terrarium use.