I did some test and found the following having SharePoint Site Collection Feature
SharePoint Server Publishing Infrastructure active and Site Feature
SharePoint Server Publishing Deactivated.
When you drag-and-drop a document to the document library with a required field, it uploads just fine without gettig the
Edit Propoerty Dialog. My guess is that you use the same method uploading up to 100 documents at the same time you don't want to edit proiperties at the time of upload. But the document is checked out to you and no one else can see it, until you check them in. And (you guessed it) you have to add the required field in order to make the check-in and possibly publish the document.
But if you upload a document using the
Upload button you get the
Edit Propoerty Dialog where you can add the required field. You also get this informative text:
The document was uploaded successfully and is checked out to you. Check that the fields below are correct and that all required fields are filled out. The file will not be accessible to other users until you check in.
If you click
Cancel in this stage, the document have already been uploaded - but are in the same stage as in the drag-and-drop feature. Even if it looks strange at first, it makes sense having this logic. You don't want to get stuck with a 100 documents pending on your actions. It's better to take care of the required field at a later stage.
In a project I experienced this with a collection of quality documents where you could bulk upload 200 documents in just two actions and later on add the three required fields missing before they where published.
Hopefully this make sense to more than me :-)