12v reading in ready mode help

Does anyone know what the normal 12v reading should be when driving in ready mode?

I bought a gauge that reads out ecu data like temp, volt, rpm, etc and it is showing that the 12v is dropping all the way to 12.0-12.1v while driving!? Intuitively this feels concerning but maybe someone can help me understand if its normal..

When I FIRST press the brake then hit the start button/ready mode, the gauge reads out 14.1v which seems perfect.. Once I put it in Drive mode/D, the 12v reading starts to drop from the initial 14.1v to what appears to be the resting voltage of my fully charged battery which is around 12.5v.

I was testing/monitoring it and when I turned on the headlights, blower motor, rear defrost, etc, the voltage began to slowly drop from the 12.5 down to 12.3, then finally going all the way down to 12.1??

I tested that the gauge reading was accurate by putting a multimeter on the 12v battery and it was....

I know that the Prius probably has advanced charging logic, but coming all the way down to 12.0v-12.2v feels abnormal...?

Also, as SOON as I hit the P button to put it in park after driving the voltage IMMEDIATELY jumps back up to 14.1v and will stay at 14.1v for an indefinite amount of time...?