Just discovered the following image of Liet Kynes:
Picture was found on DuneNewsNet.com
It is clearly from the 2021 Dune of Villeneuve and it is also visible that Liet Kynes is wearing a piece of the stillsuit covering her head, not only a mask. So, the only part that is exposed to the air and therefore water loss is the region just around the eyes.
We can therefore conclude that the Fremen in the 2021 Dune movie are wearing a headcovering to recover water from their head when their are walking through the desert with only their eyes being exposed. I was not able to find the distribution of sweat glands on the face but would think that, in order for sweat not to get into the eye, the region between eye and brow does not have a lot of glands and therefore not a lot of water is lost.
We have here a clear problem of "lore vs. visualization". From the Dune Wiki on Stillsuits:
The stillsuit typically covered the entire body, up to the neck and wrists. For the hands, gloves could be worn. However, when delicate work was performed, Fremen discarded the gloves and rubbed their hands with leaves from the creosote bush. This allowed them to work, while the residue from the creosote leaves prevented perspiration.
A facial mask could also be worn so that moisture from the mouth and face could also be reclaimed.
From this article
From the book, when Liet describes the suit:
It's basically a micro-sandwich — a high-efficiency filter and heat-exchange system. The skin-contact layer's porous. Perspiration passes through it, having cooled the body ... near-normal evaporation process. The next two layers . . . include heat exchange filaments and salt precipitators. Salt's reclaimed. Motions of the body, especially breathing and some osmotic action provide the pumping force. Reclaimed water circulates to catchpockets from which you draw it through this tube in the clip at your neck... Urine and feces are processed in the thigh pads. In the open desert, you wear this filter across your face, this tube in the nostrils with these plugs to ensure a tight fit. Breathe in through the mouth filter, out through the nose tube. With a Fremen suit in good working order, you won't lose more than a thimbleful of moisture a day...
Copied from the same article.
As for why they did not always cover the faces of the actors, see Mandalorian (emphasis by me):
Pascal hasn’t been recognized very much for this performance thus far. Though the show was an Emmy, Critics Choice, and Golden Globe nominee for Best Drama Series, its leading man has only gotten in so far at the Critics Choice Super Awards and the MTV Movie and TV Awards. That’s probably because he spends most of the show with his face entirely obscured by a helmet
From this article on Goldderby
So, if the producers / director want a shot at Best Actor emmy / oscar, they need to ensure that the faces of the actors are visible.