Limit SharePoint access for guest accounts
A guest account added in Teams gets added to the 'Member' security group for the underlying SharePoint site. The 'Member' security group in SharePoint has Edit permissions by default.
A guest using the 'Open in SharePoint' option in Teams can therefore make changes to the SharePoint site. They can add lists/libraries, delete lists/libraries, delete site pages, edit the home page. The guest account should be more limited than that... maybe just Contribute access to the 'Documents' library which contains the files stored in each channel.