In my 2009 Dodge Grand Caravan I recently had the AC compressor replaced. Since then, while running the AC the system will automatically and randomly switch to pushing hot air out of the defrost vent on the windshield. This will happen after the AC has been on for 10min, 20 min, or even and hour. Totally random.
The control buttons remain in their original AC and vent positions, but hot air is coming out of the defrost vents.
I usually shut the system down and try to restart the AC. Sometimes it works. Sometimes it doesn’t. After pushing random settings, AC on/off, air discharge positions, fan on/off, it will go back to AC operation. After a random time (15 min, maybe 25 min), the hot defrost may come back on.
I live in FL and rarely ever need any hot air.