If you are driving, there could be fresh air being forced in. This would especially be true if the fresh air vents are open, but there could still be some air being forced in, even if the vent door is closed. Just a thought.
I’m just shutting off a few individual vents when I am parked. And notice air still comes out of the vents that I turned off. Sounds like that’s normal because the ac itself is still on? On my other vehicles, which actually have buttons and louvers, when I shut vents off, I don’t notice air coming out.
This happens on some of my (non-Rivian) cars. It's fresh air coming in due to the air pressure of driving. On my Subaru, the airflow stops if I set it to Recirculate mode.
I had the same thing happen to me. I pressed the vent that I felt was open to reclose it, and it shut. So I figured it was a computer command the vent did not accept. Try that and see if you no longer feel the air coming out.
Even when I close the individual air vent, it doesn’t close 100%. It points down but there is still a gap. Not sure if Rivian air vents do close all the way?