I'm on five different Discord servers that deal with open source projects, and I'm not really that active.

That seems like a pretty significant limitation.

This is annoying especially if you want a setup where you have an internal company server and an external open source community. But Mattermost is aware of it and working on fixing it, apparently.

If it's really important to you, as a stopgap measure, you could sideload a second copy onto your phone.