12v issues

Gents, the car is Prius C Gen 1 2014. 12v battery was swapped with the noname 35a/h (not an AGM)

Two years ago after installation of the voltmeter I noticed the voltage drops. Aftermarket rearview mirror starts to blink the camera video. If the voltage is ok there are no blinkings

When the car is on parking mode the voltage is normal
Once the shift lever changed to any other mode (drive or rear) the voltage drops for 1 unit less

Measurements from the 12V battery are indicating 1V more than on other power consumers. Checked on Audio head unit plug, rear reverse lights plug and some other consumers - all showing 12,8-12,4V

14,3-14,4 v are only available on Parking mode or other modes of the shift lever with headlights on

What can be the reason?

Here is the video of how and what is happening:

 

The only problem I faced with, during these 2 years while I witness the issue is blinking small display on rear view mirror which shows the video from rear view camera
I also plan to install the audio system and afraid of the consequences of the low voltage