No highlighting on validation of multiple ordered field

There is a problem with highlighting empty required fields that are of the type multiple and ordered.

1. Edit the Newsletter Block document type in hippo:namespaces/hgge.
2. Make the field text required.
3. Save and commit.
4. Create a document of type newsletter.
5. Add multiple Content fields.
6. Save the document.
7. Validation returns 'A string value is empty', but no field is highlighted.

The strange thing is that, when trying to do the same as described in CMS7-5862, which is a similar issue, but also making the address field multiple AND ordered, I don't get the same result as described above. So there must be some additional property of the Newsletter document type, that makes it behave differently.