Workspace preview doesn't preview MM relations properly
If the workspace preview is used to preview a record of an Extbase extension having an m:n relation, the relation data isn't displayed properly in the stage preview.
The stage preview doesn't show a relation added to an item in a workspace while it is correctly saved in the database and also correctly applied if published.
So I assume the reason for the preview not working properly must be a bug in Extbase.
I created an extension to demonstrate the issue:
The Workspace preview only works in this case if https://review.typo3.org/#/c/52791/ is applied.