[Inziladun]

That's a nice find, Zigûr.

For an "in-world" explanation, I suppose it could be that Morgoth did indeed leave Beleriand for a brief time to scope out the situation. After gathering intelligence, perhaps he decided to return to Angband, where he then instructed Sauron on how to deal with the Men. Indeed, the idea of Morgoth being then bound to a "terrible" visage might give credence to the idea that if he wanted to gain something by a means other than terror and intimidation, he looked to Sauron.