1. Settings -> Users & Accounts -> Signal.
2. Click on the trash icon to remove Signal. Don't worry Signal itself will not "forget" your messages or your contacts.
3. Launch Signal. Touch the pencil icon to create a new message. Click on the "..." three dots at the top right of the screen and refresh.
Et voilà, your contacts are in sync and the ghost contact should have disappeared.
No idea how it works on an iOS device though. Sorry.
Would you have an idea how to do the same exercise on a (Windows) desktop? I will just uninstall and reinstall but if there is a less invasive solution that would be wonderful.
Thanks a lot!
Sorry, I use a Mac so can't advise about Windows, though I have the same problem on the Mac app and I'm stuck with a few old contacts that I just can't remove without reinstalling the application itself.
Except the way they manage contacts. It is a complete mess - I have contacts that changed their phone and there is NO WAY to remove them from the Signal contacts.
They are not present in any of my phone contacts but somehow cannot leave Signal.