I did some digging around on different sites and Metas, and found that this problem is extremely weird, and the behavior completely undefined.
First of all, here's a list of similar topics:
I used the tag list and the tag box on a new question on
- Arqade using "cs-global-offensive" = "counter-strike-go"
- META SE using "on-topic-definition" = "scope"
- RPG SE using "scenario-authoring" = "adventure-writing"
To see what shows up and what does not. I will mention only synonyms for the Master tag, even if they differ from the one I tried.
- "cs-": List shows "cs-global-offensive" and "csgo", Question shows nothing
- "cs-g": List shows "csgo", Question shows nothing
- "global": List shows "global-offensive", Question shows nothing
- "cs-global-offensive": List shows "cs-global-offensive", Questions shows Master
- "scenario-": List shows "scenario-authoring", Question shows nothing
- "scenario-a": List shows nothing, Question shows nothing
- "authoring": List shows nothing, Question shows nothing
- "scenario-authoring": List shows "scenario-authoring", Questions shows Master
- "on-": List shows many different ones, Question shows 6 different ones
- "on-t": List shows nothing, Question shows Master
- "topic": List shows nothing, Question shows Master
- "on-topic-discussion": List shows nothing, Question shows Master
Analyzing this, Arqade and RPG behave the same, partials matching the beginning of the synonym show the synonym in the tag list, the Question tag box needs the fully correct synonym to display anything at all. Other sides (Judaism and Math) seem to share this behavior as far as I can tell from the topics I linked.
META seems to not show synonyms in their tag-list at all, and any partial match seems to show the master, though only 6 matches are shown at any time.
IMO, any partial match of a synonym should show the master in both the tag list and the question tag box. There's no need to show something to a user that is discouraged to use/will be automatically replaced anyway.
None of the sites I looked at have that behavior, but partial synonym-matching works for the question tag box on META SE, so we know it can be done.