Same bug here, and I found the mechanism — it’s not a server registration at all. The ghost tiles come from orphaned rows in your account’s favoriteLibraries:
https://clients.plex.tv/api/v2/user/settings?key=favoriteLibraries&sharedSettings=1
Each row is a self-contained library descriptor (serverId, title, key, hubKey), so the client draws the tile without ever consulting /api/v2/resources — which is exactly why the server reads as fully deleted everywhere else while the tiles persist. It’s account-side and cross-device-synced, so app reinstalls don’t help; they just re-download the same list.
That also matches what you described about the UI: “See All Libraries” only offers un-favoriting for libraries on reachable servers, while the count still includes the dead ones.
If you tried the posted one-liner and got nowhere: the URL needs &X-Plex-Token=, not a second ?. The ? form returns HTTP 404, which makes it look like the endpoint doesn’t exist. Also ?key= is mandatory, and a POST body must be {"value": ...} or you get 400 code 1004.
I’d recommend a surgical prune (GET → drop rows whose serverId is the dead machineIdentifier → POST back) over DELETE, since DELETE wipes all your favorites. Full writeup with the exact commands here: Deleted server still shows (empty duplicate libraries) on ONE Android TV / Fire TV client only — abs
