Not all users are showing up in Talk

Now that more users are being added to the server I manage. I’m noticing that some of the new users can’t be added to existing Talk groups, nor to new chats.

How can I troubleshoot and resolve this?

Also, some users are only found when completely entering their Display Name, and then others only when entering their full Username (which when using something like Social Login is unknown to most users and way too long and random to just enter or guess.

We have the same problem on our server. We are on the latest NC version and debian 10.11 server. When searching for the user they show up however but they are not in the list of users.