Indeed, you are right.
Misteriously it was not related to the characters. If you try the topic search, it will work now. The problem was, that the search was unexpectedly excluding some of the categories. But I don't think that's the issue there with you, because you get unexpected results.
Are you by any chance using fulltext search? Fulltext mysql database queries are highly sensitive to these special characters and often they percieve them as regular expression operators. If so, then please turn off the fulltext search on the Fulltext search settings submenu: https://i.imgur.com/bmGk51p.pngBest,
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