Sony a7rii with flash onboard question

[ScoobySteve]

Hi folks I have a question regarding the Godox TT350s onboard flash. And I guess any speedlight onboard. When I have the flash turned off and the camera mode set to manual I get the settings and exposure I want and then when I turn on the flash the settings dont alter but the image in the viewfinder and screen brightens up, can someone explain why this happens please. If I turn off the flash the brightness goes back to the way it was. ???

 

[Braineack]

it's possible the viewfinder is trying to mimic the final exposure with the flash?