My brand spanking new 2018 ram with less than 1600kms on it did this exact thing. No matter what I did, floor/dash/defrost, full heat, full cold, recirc on or off, turning the system on or off, there was no change. But I could feel what little air that was getting through switching between the vents.
I know it's not the coil freezing because there's zero air flow coming from the vents and after 20 mins, I pulled off the highway, turned off the truck, waited until I could hear the blend door moving (sounds like vacuum escaping to me), started it back up and had air flowing out the vents again. It did this three times to me, twice on my way up (4 hour trip), and once on the way back (trip had more stops). But since then, it's been running fine.
From what I've been able to tell, there's a door that closes after the fan and before actual blend door.