For anyone else who needed to solve this problem, here is the solution I found to prevent/stop Cura from filling in an intentional hole in the model.
(Cura would show the Solid & XRay views with the intended hole, but the layer view would show that it was intending to fill in the ~6cm hole in my model.)
I had to enable a hidden option:
Cura > Settings > Configure Setting Visibility > Mesh Fixes (Section) > enable "Union Overlapping Volumes" & "Remove All Holes"
Now my sidebar settings show "Union Overlapping Holes" enabled by default. Turning this off showed the hole I wanted printed intact.
("Remove All holes" was already Off by default, which is what we want.)