PL9 on my Mac shows the exact same behavior as @Joanna describes.
Use the brush or autobrush tool to draw a mask. A simple stroke will do.
As soon as the mask is created the Opacity slider and the actual adjustment tools become active.
A Mask shows up in the list with the brush subtask showing active.
Look at the value of the Opacity slider. It is “0”, not “100” as expected. The mask overlay color is certainly not transparent either. It looks to be the expected 100%.
The value in the Opacity slider does not match the opacity of the mask overlay.
A user than can either toggle onto the main mask and back or bump the opacity slider to get the Opacity sider value to be in sync with the actual opacity of the mask and mask overlay.
Perhaps this bug is specific to Mac machines.