Bug. Might be fixed next patch. The mask probably doesn’t override the beard/hair, so try changing away from those.
Edit: Turns out I was wrong. There was a message on our Slack saying a very similar issue was fixed, which is why I mentioned a potential solution (which obviously didn't work as the change was intended.
"I have fixed some cases where slim armor was used if you had headwear that hid all facial hair, but had some large beard / moustache / sideburns equipped underneath. Could use some testing that everything still works."
The change was made because the mask clips with some armor, but apparently not all. I've poked art about it, but I'm not sure if it's a either/or thing.