I've noticed this too sometimes. Things I'd try:

1. AC is off or set to ECO.
2. Accelerate briskly (10-15mpg), then completely take your foot off the pedal while under 40mph, don't just back off to coast. Or back off over about a second or two then completely release. After about 2-5 seconds, the ICE should shut down. If it doesn't, try shifting to N, then back to D, with your foot off the pedal. After the MPG drops into EV mode, then you can slowly inch back on the pedal to maintain speed.

If your TCH absolutely refuses to drop into EV, then just pulse and glide, and don't stop for too long at the stop signs. Your FE won't suffer that much.