In versions of Firefly starting with release-2021.1.0 (current is release-2021.2.3), but not in release-2020.3.3 and earlier, mask plane overlays in the Rubin FITS format are displayed at a significantly lower resolution than the main image. See attached screenshots for an example.
This makes it impossible to do pixel-level investigation of the behavior of the pipelines and the reasons for mask plane choices. It's a serious regression.
The mask bits are apparently still displayed precisely in the layers dialog via DrawLayerPanelView.