How to invert a multi-component Subject mask in LR Classic

I would like to know how to invert a subject mask that has various adjustment masks (say, plus and minus brushes) attached to it. I don't see how to to this in Lightroom classic. Granted, the new masking properties are great, but if you need to adjust your Subject mask because LR didn't create a perfect one, and you then want to consolidate all of the adjustments into a new, updated Subject -- LR does not provide a way to do this. I see this as a huge omission because you can't apply new adjustments to this new consolidated subject mask. Nor can you invert this new consolidated mask. If you try -- and I did -- LR defaults back to the original faulty subject mask. It doesn't read or recognize the new mask it itself just created. 

Unless I'm missing something or doing something wrong!

So, please, if anyone knows a way to do this, kindly share your knowledge here.

If there is no way to do this, I hope Adobe will address this omission in an update to the program, hopefully, soon. 

Thank you!