"When you download the free Beard and Hairstyle DLC the growth of the beard will stop because we decided that when players want a predefined look, it would be weird to require from them to reapply it time after time."
The bug is that beard growth still occurs with the predefined DLC looks.
I wish they would simply include a special razor you could find or buy. Carry "Geralt's Razor" in your inventory: no beard growth. Drop it, sell it, or stash it: beard growth. It would be much simpler.
I've asked for it (razor) in another thread and some CDPR guy said that going to the Barber is when Geralt takes some time to relax. I believe that beard only grow after shaved clean (which I do all the time when it's grown). But with a goatie, or even trimmed short, I think it is static.