This is the thing that keeps me from recommending Mattermost in education. We had no end of trouble with MS Teams for the same reason. Students logged into our instance couldn't access their work systems. MS has made some progress to fixing this, though it has a way to go before it's as easy as Slack/Discord etc.
In my mind the gold standard is Discord. One login/password/mfa and I have access to hundreds of "servers". I have over 30 slack passwords and MFA tokens in my password manager and I know people with many more. Though I suspect the ability to self host Mattermost will make it more like Slack than Discord there.
I would also like to say thank you. Mattermost is one of the few products I've seen that doesn't lock MFA behind an enterprise subscription. I can't stress enough how happy I was to see that.