I am running LRC on an Intel i9 iMac with 128GB RAM on OS 12.5.1. In both LRC v11 and the new V12, the Mask Panel sometimes does not show up when trying to create new masks or twhen trying to edit existing masks that had been previously applied. The Mask Adjustments panel appears, but the separate panel showing mask thumbnails and related tools is missing. I can add/subtract using the Tools pull-down menu, but the full finctionality of the Masks panel is gone.
How does providing a screenshot help? The masking panel is sometimers there and at other times it is simply not there when I am trying to mask.
A screenshot will show precisely what the state of the UI is when you're seeing the misbehavior. Screenshots often give important clues about what's going wrong and what workarounds might help. Often people's verbal descriptions can fool those of us who help out here into thinking you have some other problem.
Do you have the Masks panel (when active) showing within the stack of right-side panels in Develop, or does it open somewhere else on the screen? Many people have found the latter floating / edge attached mode sometimes unreliable. Forcing a redraw / resize of the whole application window, for example by toggling whether it is "maximised", may help but my own recommendation for now is the former positioning: dock the Masks panel in below the Histogram.