Ugh! I was hoping that wasn't the solution. Any ideas why they would randomly fall off when my mesh has been rock solid for so long and nothing has changed (no new devices added)?
Initially, when I thought just one device fell off, I posted and @csteele gave me some advice on how I can use a z-wave stick and PC Controller to 'replace' the device, similar to the functionality on a C7 hub, so that I don't have to modify all my automations.
See our discussion on this thread: Modifying Network ID of a z-wave device to use a placeholder while re-pairing with hub - #12 by cj_rezz
So this is my plan to try and tackle it this way but really would like to figure out what happened!