Reactions on messages are not backed up and restored (Microsoft Graph APIs (BETA) do not reliably return Reactions)
We can only detect that a new user has been added to a chat if there are new messages in the Teams Chat since it’s last backup
Only Display name can be backed up for Tags used within Group Teams channels
[Applies to Customers in the Europe (Netherlands) datacenter] All chat messages that form part of an ongoing Teams conversation will be included in a user's backup, including messages posted before the user was added to the conversation, regardless of the share options selected while adding the user
[Any Customers NOT in the Europe (Netherlands) datacenter] When a user is added to a Teams conversation and is granted visibility to previous chat messages; these messages that had been sent previously might not be backed up.
Files sent over chats will be backed up first time round, if the file is later modified via OneDrive and not re-sent on the Teams chat this will not be detected during the chat backup. This will however be backed up via the OneDrive/SharePoint backup
Files shared by an external user are not backed up as these files are actually links to files inside the external users organization
If a chat is initiated by an external tenant user, we will only be able to backup all messages from the time that the backed-up user is invited to the chat onwards, even though the user is granted visibility of previous chat history.
Microsoft M365 Limitations
For mailbox backups, if a custom folder is named "root" Items placed in such a subfolder will be excluded from backup. This is native behaviour from Microsoft's APIs and cannot be changed. As best practice, always avoid using "root" as a folder name.