bumping this thread to report what I've figured out now about how my game's handling volcano and flood graphics
The following seems to be very consistent: the first time I load a save file after launching the game, the graphics appear as expected, but any subsequent loads will result in the graphics being missing. The graphics return when I exit the game entirely and relaunch.
In other words, it seems that pressing "exit to main menu", or loading a previous save after already having a game loaded up, will both disable flood and volcano graphics for the rest of the time that I have the Civ 6 application running.
really sucks for people like me who like looking at the graphics and who also like reloading saves to undo mistakes :/