Masking is whiting out the whole image. Aug 2022

The O key toggles on and off the masking extent preview. So at a given moment, we are either reviewing where we are masking, or else previewing what local adjustments will continue to show when we exit the masking mode. 

There is also a "Show Overlay" checkbox in the masking panel, equivalent to pressing the O key.

and the three-dots menu next to the red square in the screenshot, gives access to different mask preview modes. The usual default is "color overlay" and in this case Shift+O cycles through different standard colours for that (or the color square lets you choose your own). Other mask preview styles will (for example) match what the mask thumbnail shows.

The same options menu lets you access settings where you can reverse whether the mask overlay should show in the places that are going to get the adjustment, or happen in the places that are not going to get the adjustment - as you prefer.