Here's some interesting data. At 61 mph on flat ground, good weather and not significant wind:

Streaming 5 car lengths behind a traditional semi, I get ~44 mpg. But behind a flatbed I get up over 50 mpg.

 

Is not 5 car length is too close at 61 mph? It is more like a 1 sec gap.

 

Interesting.

The flow beneath a large truck can have a pronounced effect, esp. since a car is mostly 'under' the trailer's level.

At 5 car lengths back, the draft effect is more due to the truck's slipstream rejoining into smooth, fast-moving flow than it is due to vacuum/sucking as it would be in a close-in draft. So the car has a smooth airflow that is moving relatively quickly. A flatbed would have smoother flow (less turbulence and vacuum in the 1st place), whereas the semi's stream still has a lot of buffeting and the car needs to fight that.

Think of a boat wake- there's a hole immediately behind the stern, then turbulence, then very calm smooth flow several yards back. The bigger ther boat, the larger the hole and turbulent region.

We call it all 'drafting' but the mechanism causing it is different.
That's my guess as to what you've just seen.

 

Drafting can mean mpg improvements, but I've never understood the logic. For a few dollars saved per year, there's a BIG risk of a broken windshield, chips, and dents. (Trucks throw lots of rocks and debris). Fixing some of that, one time, will cost more than any gas savings. The truck is also a visibility barrier, causing a big safety problem. I try to stay well away from trucks, following further away than I would follow a car.