Lightroom Classic v 11.2 invert custom mask

You cannot invert an entire Mask but each 'layer' nested inside a Mask does have its own separate Invert option, as well as other controls. Between these, any conceivable combination of the different kinds of selections can be achieved AFAICT.

(For example) you can select Subject, then invert that 'layer' so that non-Subject areas are selected instead. If that original auto Subject selection had wrongly strayed into some areas you consider to be Background, then you would make an Add brush over the top, and paint those areas back in  - so your finished overall Mask selects for the entire background.

Watch this video, well worth the time to learn how the new masking controls in ACR and LR work:

https://www.youtube.com/watch?v=GDg6FdNdufs

Inverting of the Total Mask is a promised future feature but until then you can invert the total mask by the following technique.

Invert the Bottom Component  (your Select Subject)

Toggle the +/- of any Component above that (change your Brush from subtract to add)

My desire is to effect the background of my image.  I have used the new "subject detect" mask but the result included too much of the image.  I added a (-) brush to eliminate the excess but the invert mask option has disappeared from the control panel.  Right click on the mask reveals no invert. Someone suggested control "i" but, of course, that just toggles the info.  Has anyone experienced this or know the answer?

Thanks