Thanks, papbot, for your suggestion.
I’m running iOS 11.2.1 now. Unfortunately neither this update nor device restart solved this little bug.
I’ve just figured out a solution:
1 - turned off iCloud sync for keychain, and chose delete all password.
2 - the re-appearing entry was still in the list, and still couldn’t delete it.
3 - iPhone backup to iTunes.
4 - ‘Reset All Settings’ on iPhone.
5 - restored backup from iTunes.
Cheers!