On Ask Ubuntu, in the retag dialog, I noticed:
Notice that [fontconfig] shows up as both a synonym and a regular tag (which somehow has 4 usages). Clicking through "fontconfig" shows it does seem to be an ordinary synonym.
As I recall, normally if you start typing the name of a synonym, only the synonym target is shown up (with the name of the synomym bolded/highlighted); the synonym shouldn't be displayed as a separate tag.
I presume the bug comes from "fontconfig" still having usages, even though according to What are tag synonyms and merged tags? How do they work?, synonymized tags are supposed to always have 0 uses.