1. Their is. or was, a bug revolving around LrC, and MACOS where if the Mask Panel was located withing the Lupe view as opposed to under the tools bar just under the Histogram area, and you maximized the window, the mask panel would hide and not come back. Solution was to place the mask panel under the histogram area, then maximize the display, after which you could move the mask panel back to over the Lupe, and it would behave. Supposedly this was fixed.
Mask Panel in loupe area
Mask panel under tools below histogram
2. There is a know behavior where if the mask panel is in the Lupe view, it will temporary disappears when moving a mask adjustment slider. This is by design as to reveal anything behind the panel.