RE: Button on Ribbon not showing in Ribbon Workbench
If I understand well, "Done" is your custom button, and you've added it through PowerApps.
Could you please also confirm if the button is part of a managed solution or an unmanaged solution?
If a custom button is added from PowerApps, it will not appear in the Ribbon Workbench, and vice versa.
We have also observed the same behavior.
So you can use either PowerApps or Ribbon Workbench, but not both at the same time.
Hope this helps.
Thanks!