Do beards actually break the seal of gas masks?

No they don't break the seal as the beard can prevent the seal in the first case. I have seen it with dependents one time when we had a come to work day that included the gas chamber I ran at fort carson in early 80's as a junior officer. All that had facial hair were told the if they went in the chamber there was a high probability the mask would not work and the would experience the CS would give them fully. Of the 15 with facial hair. 10 choose to give it a try. All 10 left the building in a hurry coughing and crying like babies. Now the bears were of various lengths. Of which 3 were nearly trimmed. And personally sweat and smooth rubber provide a great seal.

I wouldn't want to find out, during an emergency! The ones we had in Basic didn't seal very well to bare skin because they were old and needed replaced.

Certain career fields require no beards due to health reasons to prevent hair and or flaking of dry skin falling into food or open wounds during surgery. After 20 years in the Air Force sporting only a mustache upon my retirement I started my beard only to have to shave it off as a requirement for a job requiring a facemask due to movement of polyvinyl chloride liquid moving from a train rail car to a holding tank. That job lasted a year upon which I started to grow my beard again, only to shave it off as a flight paramedic. From there to the VA Hospital in the Surgical supply department. Upon total retirement I finally grew my beard 16 years ago. It shows on my picture.