So I still have the problem of random project folders just disappearing (it happened again, with the reconstructed version of the last one it happened to). But now I also have a recent project that didn't disappear, but four of the seven pages in it give an error message when I try to open them: "...Cannot open file. File is probably opened by another application." Then it offers me three choices: Rebuild, Delete and Rebuild, or Do Nothing. None of them result in my drawing returning to me. Anybody have this issue? Any idea what causes it and how to fix it? (Ideally a method that recovers all my work)
So it turns out this was caused by a desynchronization problem from having the project folders on the network while the application folder was local. Why they got changed is due to some of the settings resetting during upgrade from 2017 to 2018 but not all. The folders never went away, but the project list was pointing at old folders that no longer existed. Once the database settings were corrected, everything was where it said it was. The only difficulty was merging the project created during desync.