From what I've seen and read, it appears that in the 1300s a lot of the knights and MAAs wore open-faced helms (pointed helms), and more mail armor, with some plate over that - Crecy and Poitiers. Of course, I suspect some would have face coverings of armor too, since they did all the way back to the different Crusades eras as well.
Of course, with the various drawings, and/or paintings, perhaps the artists were deleting the face coverings on purpose to show the expression on the men's faces, and taking free license to do so. I have read that many artists tended to depict men in their current day's armor, instead of the more accurate armor from the time of the actual battles they are depicting, on many occasions, not having been there to observe those first-hand, which can be problematical too.
"A hounskull, called in Germany a hundsgugel, was a form of steel helmet worn in Europe in the Middle Ages, almost invariably by knights and other mounted men-at-arms, from the middle of the 14th century until approximately 1420. It offered extensive protection for the wearer's face at the cost of some visibility, but its distinctive visor could be raised or lowered at will". It's also referred to as a pig face helm.
From around 1400 on, plate seems to become more and more predominant.
I'm no expert of course, but that is the overall impression I've been left with, by reading some of the above, and looking at various photos of the periods in question.