However, when I display the list of tags and I type the name of this tag in the search box, it is not shown. (See the screenshot below.) The two other synonyms of scope do not behave in this way. (EDIT: As noticed by Martijn Pieters, (where-to-post) and (allowed-topics), which are the other two synonyms, also disappear as soon as the first character after the dash is typed in.)
Maybe it has something with the form of this tag and with the fact that it is a synonym.
When I tried to search for the tag (on-topic) on meta.unix.SE it worked fine. (That tag is not a synonym of another tag on that meta.)
Another interesting thing is that when I start typing a new question on meta.math.SE and I start typing "on-topic" into the tag field, no tags suggestions are shown until I write the whole phrase on-topic. (Usually most popular tags containing the part I have already typed are shown.)
The following screenshot is from meta.math.SE: