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 a

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.