TYPO3 Core

How I reproduce this:

1) Add new Content Element "Menu > Pages"
2) Select a page through wizard.
3) Click "View"

The modal is not shown. The preview is opened in a new tab. The form is submitted. The record is not saved but still "NEW" in the bottom right corner.

You can repeat as long as you want, the record is not saved, nor a modal is shown.

Until you fill in something within "header" click on "View". Afterwards you can remove the value from "Header" and it works as expected.

I can reproduce this with the above field and flexforms.