@GuidoD, can you check via the developer console the HTTP request being made when typing that @? The frontend should hit /api/v4/users/autocomplete and get up to 25 users returned:
If that response is empty when you get no mentions, it helps pinpoint the issue server-side. Do you use ElasticSearch and have Enable Elasticsearch for autocomplete queries enabled? (Or Bleve, and use the same?)
If the response contains entries that then don’t show up client-side, it helps pinpoint the issue client-side. Do you see any console errors at that point?