2013 Sentra AC Issue

The AC in my 2013 Sentra sort of works sometimes but never gets really cold anymore and when the car is stopped it gets warm. It has almost 100,000 miles on it and the problem began right after I got the transmission replaced. I’m not sure if that’s related or not. I brought it to Firestone and they checked it for 3 hours. They couldn’t find anything wrong with it but agreed that it is not working right. They changed the cabin air filter and topped off the Freon which was low but not super low. It still doesn’t blow cold air like it used to and you have to drive for a good 10 minutes before anything close to cold comes out of the vents. It goes from hot to moderately cool back to warm and when stopped it gets super warm. The mechanic took me for a ride to show me that the air flow actually decreases when the car is moving. He said he’s never seen this happen in 25 years of working in cars. Generally when in motion the air flow increases. So less air flow when moving but slightly cooler then when stopped. He said he tested everything and everything is working. The scary thing is that bc the AC doesn’t get cold enough, I’m driving blind whenever it rains bc I can’t defog my windows. Has anyone had a similar issue and gotten it figured out? I’ve seen elsewhere online where others have had this exact issue and nobody seems to be able to figure out why. I live in SC and a working AC is essential.