My web browser will not display attached files. If you include your screen capture by clicking on the "Insert Photos" icon, then I can see it.
Are you looking at the top level of the mask? There is no Invert under the triple dots. You have to go down one or more levels to sub-masks to see Invert under the triple dots. In the screen capture below, you need to click on the triple dots where the Green arrow is pointing to, not where the Red arrow is pointing to.
You could also use the shortcut key: apostrophe.
oh jeez... that feels unintuitive but your diagram is perfect. Yeah I guess I was looking at only the red arrow level?
yes, it's definitely confusing and also missing an essential feature. Here's the FR to have Invert also added where the red arrow is:
on the right of the UI, yes, but it's to the left of the area that you circled.