A couple suggestions, the first being to find a new dealer service department. Your vehicle obviously has a problem according to the way you are describing what it does.

The second suggestion would be to document exactly what you do for an entire tank or a week or whatever appropriate time period you think is necessary. This will be a major pain but so isn't not getting what you paid for in a vehicle. Keep track of where you went, how long the engine ran before going EV, the outside temp, a/c usage, traffic, distance travelled, etc. Put down anything that could possibly vary day to day or trip to trip.

Take your logbook to the (new) dealer and show it to them. Make them perform a diagnostic on it and give you a printout showing no codes. Ask them if the computer can be reprogrammed. It certainly sounds like some sort of program error or a bad sensor of some sort. Any error like this should show up on a diagnostic.

If I remember correctly from another thread you are getting about 24 mpg or so? My 2005 is averaging 32+ over 5000 miles and the first tank on my 2006 I got over 31.5.

Good luck.

Ray