I have three layouts that are associated with my *.qgs project file. Each layout includes at least one map and a number of text boxes that serve as captions for the map(s).
After a few days of formatting, my layouts are almost completely finished. However, some of the maps on two of my layouts no longer display as I edit the layout (instead displaying "Rendering map" on a completely grey background). For earlier versions of QGIS, the problem may have been that I toggled "Cache" or "Rectangle" instead of "Render," but these options do not display in this verions of QGIS (Girona, QGIS 3.0).
Importantly, if I try to delete any of the offending maps, QGIS hangs and I have to force quit or kill the program. Also, I should mention that the offending maps successfully render when I export the layout to a pdf... I just can't see them while I'm editing the layouts.
UPDATE: I upgraded to QGIS 3.8 (Zanznibar), and haven't had the problem since.