Examples:
https://forum.monocycle.info/search?q=%40MouvementOndulatoireUnifié
https://forum.monocycle.info/search?q=%40Aurélien
Search with usernames with no special characters work:
https://forum.monocycle.info/search?q=%40canapin
https://forum.monocycle.info/search?q=%40bouin-bouin
I’m using Discourse 2.9.0.beta9 (c245b74398)