I administrate a Mattermost Team Edition server and have just received a report from a user.
Windows Defender reports the current version of the Mattermost Windows Client which is available in the Microsoft Store to contain a trojan.
Since this report is only coming from one user and I have had no trouble on any of my machines, I assume this is a false positive. I will investigate and come back with details later.
Windows Client Version: 4.0.1 (seems to be the current in the Microsoft Store)