TYPO3 Core

I have the same problem in Typo3 7.6.18.

TCEMAIN.preview is configured for the News extension as described in the News documentation.

In LIVE a click on "Save & Preview" correctly displays a News preview in the frontend.
In a WORKSPACE a click on "Save & Preview" on a News record with unpublished changes fails. The preview uses the record UID of the Workspace record instead of the UID of the original.

One possible solution might be some code for handling Workspace records in TYPO3\CMS\Backend\Controller\EditDocumentController:generatePreviewCode()

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55070

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55070

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55070

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55070

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55070

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55070

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60333