This issue affects migrations done with the ShareGate migration tool 15.3.0 (released on 11/23/2021) to the ShareGate migration tool 15.3.3.
If you performed a team migration with one of the affected versions, you could have conversations in the Posts tab from your private channels accessible to all the members of that team.
When a new team is created, a site collection gets attached to that team and all your channels share that site collection for their content with the exception of private channels.
Private channels get their own site collection with different permissions to ensure only users with access to that channel can access its content.
With the versions above, your private channel's conversation history would get copied in the teams' main site collection instead of the private channel's site collection.
Though the private channel's conversation history will not be visible directly, any user of the team could navigate to the attached site collection and find them in the Team Message History library.
For more information on how Teams connect to SharePoint site collections, see The relationship between Microsoft 365 Groups, Teams, and other services.
Make sure you have the latest version of the ShareGate migration tool. A fix was introduced in version 15.3.4.
To fix this issue, you will need to find your affected teams, perform incremental migrations to copy the message history to the right location, and finally you can delete the message history of your private channels from the main site collections of your teams:
Find your potentially affected teams
- Select Tasks under ACTIVITY in the menu.
- Click on the Add filter dropdown.
- Select Start date.
- Input dates from November 23 2021 to the date you updated to ShareGate Desktop15.3.4 and up.
- Click Apply filter.
- Click on the Add filter dropdown again.
- Select Task type.
- Select Copy teams.
- Click Apply filter.
- Take note of all the teams you migrated within the filtered tasks.
With private channels from a custom report
- Select All reports under REPORTING in the menu.
- Click on Create custom report.
- Select Microsoft team as your Object type.
- Make your filter read, "Private channel count" "Greater than" "0".
- Click Continue without saving.
- Select your tenant.
- Click Next.
- Click Run now.
- You can click on Export in the top right to get your list of teams in an Excel format.
Perform an incremental migration of your teams
You can perform an incremental migration of your private channels to get their message history migrated to the right place.
Note: The incremental migration will not remove the message history that was migrated to your teams' main site collections.
Delete the message history for your private channels
Note: Before you delete the conversations, make sure they were migrated during the incremental migration phase if you are required to preserve them. Otherwise, you can make a backup before deleting the message history at the destination.
With your list of teams, you can access the associated site collections to delete the message history for your private channels.
Directly in Teams, you can click on the general channel of the affected team and select Go to SharePoint from the dotted menu to access the site.
Once in SharePoint, Locate the "Team Message History" library. The library will contain a folder for each separate channel.
Delete the folders with the same name as your private channels.