OK, adding a canvas does work, thanks. I had tried this in the past because I don't like the idea of everything re-rendering on canvas rebuild; but it had caused a lot of bugs, so I went back to a single canvas (I don't have any performance issues, anyway). In my brief testing here, however, it doesn't seem to be causing any problems for the map view to have its own canvas.
I redid the previous test, this time keeping all the hexes enabled while adding over 1024 images to the map container. The result was that no graphics disappeared. So my question is: why does this only happen with STM quads?