Federate a private room from Element to Mattermost does not work

[muratbayan]

refers to feature added in https://github.com/mattermost/matrix-as-mm/issues/14

I followed the documentation to create a private room from Element and have Mattermost users in there.

• Create the room without E2EE
• Invite Mattermost users in Element and the mattermost connector user
• Mattermost users look like it has accepted the invitation, but the connector does not look like it has accepted the invitation As a result no room created for the Mattermost users in Mattermost and no messaging across platforms

@janostgren