Welcome to the forum.
MAX AC does two things, it closes the Fresh air vent and closes the Valve on the heater hose so heater core in the cab is cut off.
That's the only differences between Regular and MAX AC, on most vehicles it just closes Fresh Air Vent.
On Rangers there is a Blend door that separates the heater core from air flow, this door is operated by the Temp Control, COLD-HOT.
Blend door problems are a known issue for Rangers, and most Ford Trucks.
It uses an electric motor not a vacuum motor like other doors/vents in the dash.
When AC is on the Blend Door should close and cut off air flow thru heater core, it reads like yours is not closing.
With MAX AC the coolant flow thru the core is cut off so blend door position wouldn't matter.
Google: Ranger Blend Door
I would do this test first, find the heater hose valve in the engine bay, it will have a vacuum hose attached to open and close this valve, with engine running check its position in regular AC and MAX AC, see if it is open and then closed.
You can feel the heater hoses get colder when valve is closed and heat up when it is open.
If valve closes only when MAX AC is selected then I would suspect Blend Door issue