Brush issue with selection

It looks like you may have luminosity selection active.  Photoshop only shows marching ants around the pixels that are more than 50% selected.  All pixels may be select at some percentage.  Save the Active selection as an Alpha channel and look at the Alpha channel thumbnail the get a better view of the selected pixels.

You normally adjust luminosity selections not use them as painting guides. To work smoothly on shadows and highlights

You are painting with white with 30% opacity on the mask and it is partially (30%) revealing the layer underneath. If you are trying to do something else, let us know what you are trying to achieve.

How did you select? Is there feathering?

~ Jane

Are you trying to paint on the image, or on the Layer Mask? What does the "Bright" Layer Mask look like in the Channels panel after brushing?

Did you created your selection using channel, like Ctrl + click on channel? Try creating selection using Lasso tool, for example, or Color Range.

You are not painting full white color but rather gray or white with transparency, right? When using channel to click and select many pixels will be semi-selected or somewhat selected, you can paint on them but not full black or white, only percentage of it based on grey value on channel from which selection is created.

Tried checking build from selection, on the build and modify mask section?