In your first mask, on top of the list of filters, Operation needs to be set to Overwrite
Add your other masks just bellow the first one and set Operation to Add.
I used Brightness to hide the subjects, but of course, you can choose any filter of you want there. Blur, Mosaic, etc…
Worked to me perfect … thanks … maybe some day we can use different effects in this masks !
Another strange is I needed to hire the uppper video track and all set and showing the image of upper track … crazy but the important it worked fine to me in the final !