Flexform is not rendered in content elements, when list_type is not empty
When you have custom content elements using flexform (DCE in this case), which were plugins before, the list_type property may not be empty. In this case the flexform is not rendered properly anymore. Instead you get this:
You can simply reconstruct this issue, by adding any value to list_type property of a content element and edit the item. Of course the content element needs to use pi_flexform.
This issue appears in TYPO3 7.6 and 6.2, so it's not a "new form engine" issue only.