Display remains on-ish

Not many people on here have a Prius that new... But try disconnecting the 12v battery for a few minutes. That will reboot all the systems from factory default settings and usually eliminates a strange glitch like this.

 

If you drive 90% electric, this means that you charge the car often and while charging, the 12V battery is also being maintained. So no, I don't think that's it. More probably a bad battery.

 

The 12V is being charged whether running electric or on the engine. It is possible the trips are too short to fully charge the battery back up from the starting event though. Lead-acid life spans become shorter when they aren't kept at 100%. The mileage on the car points to short trips, plus it means longer periods for vampire draws and self discharging to further lower the SOC.

The battery is maintained while actively charging. When the traction battery charging is done, that maintaining ends, and the vampire drain on the 12V is higher than when unplugged as charge system monitoring is going on.

Hybrids don't use the 12V to start the engine; just boot up the computer and flip some relays. So the manufacturer puts in a lower capacity battery to do that job. There still is all those vampire drains cars have these days though. If the battery is a starter type vs a deep cycle lead-acid, it is further susceptible to being to deeply discharged.

 

Yes, I usually unplug after the car has finished charging to prevent vampire draw, although from what I read, is most problematic for the newer Gen5 Prime.