Kodi has started offering updates which do not exist in any installed repo. Removing the add-on and reinstalling it does not resolve the issue. I have previously encountered this on Kodi 17, and it still happens on Kodi 18 release candidates. I do not understand the mechanism which Kodi uses to build the 'Available Updates' list, but it seems like some kind of sanity check is needed here to verify that updates are actually available in some installed repo before offering them to the user.
1. Until this bug is fixed, can someone explain how to manually purge the update catalogue and force it to refresh ?
2. Why does it say "Repository: Unknown" for practically every add-on? Is this a bug, or an incomplete feature that is still in development ?