@Canapin I looked into this, and I think it’s behaving the same as other avatars in Discourse. If the prioritize username in ux site setting is enabled, it will use the the username. Otherwise, it will use the Full Name.
Can you double check the value of that site setting on your site? Here on meta it’s disabled, and so it will show the full name on mouseover.
Yeah the lack of user cards has frustrated me as well! Let’s fix that: