2013 Honda Accord EXL 2.4L 130,000 miles

I have had this car in the shop twice for this issue and the professional Honda mechanics swear there is nothing wrong.

My Accord heats up as normal and the heat responds to the temperature I set it on. It blows hot air the way it should. However once the engine heats up, and I have been driving for about 20 or 25 minutes on the highway, the temperature of the heat drops to barely lukewarm.

Even if I crank the temperature to high, the air remains lukewarm. The engine is not overheating according to the gauge, and I have not been able to find any sign of a coolant leak. There are no other messages or indicators.

Has anyone experienced a problem like this or have an idea as to what might be the problem?

 

Maybe there is an issue with the blend door under the dash. Do you get the same temp on the passenger side?, since these cars have dual climate zones.