I thought this post never made it to the forum when the server seemed to be down, but now that I see it did . . . I later found that under certain circumstances, the underscore results in FolderItem.Exists being False. I will narrow this down soon and post.
I use suffix like _Name (String: Name of the file data base file), _FI (Name of the FolderItem Reference), _DB (Name of the Reference SQLite Data Base).
No trouble on macOS (El Capitan and previous), where I create and build projects, nor in Windows 8.1, Windows 10 (and prior that Windows XP) where I run the application. Linux Mint is OK too.
Might the issue be that the underscore in the code is a plain ASCII underscore, but the file name uses some weird Unicode character that looks like an underscore? Or vice versa. This kind of thing has bitten me in the past. E.g. hyphens and m dash, etc.