Inverting a selection in LR Classic

Sorry Brode2 I should have looked at the date of your post - I'm refering to Classic 2022

Have had some odd issues but think most are down to finger trouble. I have been using dehaze but find texture  and clarity are gentler on backgrounds - a bit like the tilt shift filter in PS - first select subject - mask 1 and adjust, then apply a linear gradient - mask 2 dehaze etc to blur (or any adjustment) select subtract and select subject removes blur from subject. The only real issue I have found is inverting a subject mask modified with brush additions and/or subtractions.

You need to apply a separate brush.

A very late reply: The Edit > Invert Selection does something completely different from what you expected. It selects all the unselected photos in the curremt folder or collection and deselects all the currently selected photos.  

It isn't possible to invert the masks of local adjustments, except for radial filters. Many people have requested this over the years:

https://feedback.photoshop.com/conversations/lightroom-classic/lightroom-classic-id-like-the-ability-to-invert-a-mask/5f5f44f74b561a3d422c74ea 

Please add your constructive opinion to that feature request and be sure to click the Vote button at the top left and Follow at the bottom of the first post. That will make it a little more likely that Adobe will consider implementing the feature and you'll be notified when they do. Product developers rarely participate in this forum and won't see your feedback.