HVAC Recirc issue/question

Driving to work the other day and I followed a 70/80s K5 Blazer (?), it was on 35s Super swamper boggers(?). Lifted with what looked like 10" blocks. What a POS. I think it must have been carbureted and tuned poorly to run a 6'K. Exhaust coming off that thing was so terrible and filled my cab with super RICH exhaust fumes. Couldn't pass it due to traffic, and the fact that he was trying to hypermile the truck. Speed limit 55 mph. He would get down to 45 and as I approached he stomped on the gas and accelerated up to 65, and then coast back down to 45. Every time he stomped on it a plume of smoke would leave his tail pipe, almost like he was rolling coal but with a gas engine.

I tried to turn my HVAC to recirculate so that I did not have to smell his exhaust but it seemed like it was still letting in outside air. This event inspired me to change out my cabin in air filters.

Last night I went and change them out and decided to do some poking around. In most cars, including our Frontiers, the air door between the outside and inside air sits right above/next to the air filter compartment. With the glove box down you can hit the recirculate button and watch a panel open and close. On my truck with the air directed at your feet only the recirc door does not open, but the button lights up. If I flip the air to feet and face or face only the door will open. The temp was on max heat. This must have been what was going on, as in the winter I generally only use the defrost/feet and feet only as it is only 1 click away from easy other. Is this how it is set up or is there something wrong with my HVAC system? I did not try to use recirc and cool air on feet only. In reading the owners manual it recommends that recirc not to be used with heat. But why would heat and face allow recirc and heat and feet not?

Anybody else have knowledge of this seemingly obscure issue.