This is most likely caused by formatting (HTML, etc.) between the characters.
Anki only searches within formatting in the sort field.
From the Searching section of the manual:
Anki is only able to search within formatting in the sort field you’ve configured. For example, if you add “ exa mple” to one of your fields, this will not be matched when searching for “example” unless that field is the sort field. If a word is not formatted, or the formatting does not change in the middle of the word, then Anki will be able to find it in any field.
When in the note editor, press Ctrl+Shift+X to open the HTML editor and inspect the field content.