Required HTML validation doesn't work properly (only after document is saved)

HTML/Rich Text field mandatory validation behavior (in short):

• when document is created, it shows required error as it should
• once you fill in something and remove it (completely, e.g. via source markup) it stops working

After document save operation, it starts working properly.

STEPS TO REPRODUCE:

The document has a mandatory text-field (headline)
The document has a mandatory rich-content-field (copy)
The document has a mandatory image-compound

Take these steps:

1) Insert an image and save the document (2 errors are shown, since 2 mandatory fields are not filled)
2) Fill in the copy and save the document (1 error is shown, since 1 mandatory field is not filled)
3) delete the copy and save the document (1 error is shown, though 2 mandatory fields are not filled)
4) insert a headline and save the document (0 error are shown, though the deleted copy form step 3 reappears in the copy-field and the document is saved)

Once the document has been saved without errors, the behaviour doesn't happen anymore.