When I've tried to register a new username with the non-ASCII acute e symbol, the web UI didn't show me an error message but instead created my profile with an underscore (as you can see). <span class="h-card"><a class="u-url mention" data-user="+Bonfire_Feedback" href="https://playground.bonfire.cafe/pub/actors/Bonfire_Feedback" rel="ugc">+Bonfire_Feedback</a></span>