I must say it was very confusing.
The main issue of the problem, was that, when P3D V4 was released last year, we switched from installing INTO the sim, to installing in a different folder and, in order to SAVE users from the hassle of having to uninstall everything (FSDT and Flightbeam), re-download everything, and reinstall everything, just to move files they already had from a place to another and reconfigure the sim to see them, we had the "migration" process, which did all of this automatically.
The migration had a side effect: the Uninstaller wouldn't work completely, because an uninstaller it's made at *install* time, so it can only remove files from the original locations they were installed into, but they were moved by the migration process, and it wasn't possible to update the uninstallers too: you need a new installer too, which would in turn create an updated uninstaller able to remove from the new file locations.
This wouldn't be a problem, and it's not a problem for our sceneries, since user would eventually get an updated installer anyway, but it became a problem since the new installers from Flightbeam don't use the Addon Manager anymore, so you must do what you were originally saved from doing by the migration: uninstall everything, and reinstall everything only using updated installers.