"Screen" and "LED" brightness settings change to dim when using side power port

Every time I power it on the brightness settings have to be readjusted. Anyone else have this issue?

I’m using a power supply rated to 2A and outputting 9V which is well within the specified “4–10
V DC”.

It is probably nothing more than a power saving feature since the port is originally intended as a battery port but I want to be sure. It would also be nice if there was a way to overide the unit defaulting to the dim settings when using the side port.

Yes it is.