I have the same issue here! i see this since a week and it´s something happening in the browser-edition.
use it on a mac with chrome.
@frequently, not allways, it happens that i can´t see somebody when i try to use @mentions in a channel. if i look in the team-members i see them, but still back to @mention i don´t. sometimes it´s gone after i change to another team and come back, sometimes not.
@This bug starting to hit my users as well. Hopefully it's not a design change but @mentions do not seem to be working unless you are replying only and not in new conversations.