This bug applies to any page with a tag editor (ask question, edit question, edit tags, etc.). The synonym tags do not show up in the list of available tags when the name is partially typed.
Using assassination-classroom as an example, it currently has ansatsu-kyoushitsu as synonym. However, ansatsu-kyoushitsu doesn't show up when ans
is typed in the tag editor.
This totally defeats the purpose of having Japanese tag synonyms, as they are supposed to help user to find the correct series tag.
The desired behavior is the current behavior in the search box of tags listing. It turned out that it is also buggy.
While the search box works with ansatsu-kyoushitsu, it doesn't work with the-legend-of-koizumi when searching for koi
(it is a synonym of mudazumo-naki-kaikaku).
anime
, [retag] comes up, but for your example and forattack-on-titan
it definitely doesn't work.