Expected volts on Battery when running on idle

Subaru Forester 2003

I replaced my battery a few days ago.

This morning with a multimeter (Car was sitting overnight), I saw that the volts on my battery was roughly at 12.54 which is good. (Car not running)

Checked the BAT Terminal on my alternator, roughly the same (also good)

Started the car, the BAT Terminal reported 14.6 and my battery was also reporting roughly the same, after a few minutes, I began opening headlights, defrost, wipers, heaters.

The car battery voltage began to drop 13 and lower, the second I provided some acceleration, the battery clearly went back to 13.99 to almost 14.2

All that time, the alternator was still registering 14.4 to 14.6 volts

To me, this seems to be working as designed. In an idle state with no gas and all bells running, I bel it is normal to be hovering at 12.5. (Again Idle, with no gas what's so ever)

Just to make sure, I had the alternator tested by a manufacturer and he said that the alternator is fine.

My local mechanic keeps insisting that even on idle (no gas being applied), with all bells running (Heater, defrost, radio.) We should still see a battery with a voltage well above 14.

How can this be? Can anyone confirm either way please?

I really need some sort of official statement, to me, having the battery always at 14 volts (without any acceleration being fed to the system) is not normal, what I have is normal, but my local mechanic says otherwise.

I look forward to hearing from the experts

Thanks Again