I have for several customers started to work with shared channels in Teams. We have been using the B2B Direct connect config.
One thing I saw and thanks to MVP Mårten Hellebro and MVP Luca Vitali for great advice to fix the issue.
- First was to always when run shared channel have atleast two owners of the Team.
- Check Sharepoint. Make sure to add the onmicrosoft.com and company.domain
Then Save and you should be good to go.
This saved my day and it all worked for my customer.
This is how I did the SharePoint Config
In the admin page for SharePoint we goes to Policy-->Sharing
Then add your domain who you want to have this with.