Mentions are not working for subscribers, only administrators

Hello

I am trying to use comment mentions on a forum on my website. If I am logged in as admin and access the forum I can tag members without an issue, however, the users of my site have to be logged in in order to access the forum. They have a user role of subscriber and a forum role of participant but typing @ is not bring up the list of users on the site to be able to tag them.

Are there any restrictions around the subscriber / participant role?