I've seen it tested, and it doesn't really. I have no idea if there's studies on that but, like you said, there isn't evidence, so why would we have this restriction? If they actually cared about this reasoning, they'd put money into testing it. However, disproving it would remove their ability to use it to justify exclusion, if this is the reason instead of functionality.

Just think about it. If you put enough force on beard hairs they are pretty firmly against the skin. They don't push the gas mask away. There aren't really going to be any gaps. It just doesn't logically make sense if you consider it for more than a second.