Private channels in Teams are not backed up by default because Keepit cannot automatically add the Global admin to the channel to get access to the data. The Global admin is the Microsoft service account used for the backup, і.е., the service account used to authenticate a connector.
To back a private channel, the channel owner, which is the person who created the private channel, must make the Global admin a member of the channel.
To add the Global admin to the channel:
1. In Teams, open the private team channel. Private channels are marked with a lock icon.
2. In the upper-right corner of the Posts window, select the people icon.
3. At the bottom of the People window, select Add members.
4. Enter the email address of the Global admin and then select Add.
The private channel will now be backed up and the channel date be in Keepit after the next backup is completed.