Several issues can be the cause of bearding...
Lack of space in the brood nest
Not enough to do for the bees
Not enough ventilation
Crowded brood nest in bottom box
Other reasons I'm sure
When that happens to me, it's usually crowded brood nest in the bottom box. I always just add a box of drawn comb to the bottom of the hive, not the top. It gives the bees somewhere to hang out when they don't have enough to do or when there's crowded broodnest on the bottom. It clears up the bearding, giving the bees somewhere to hang out and something to do also, cleaning out the cells and whatnot. It also gives the queen room to move down if she wants to expand the nest, and does not disturb the way she has the hive arranged, can still keep the honey and stores on top and around the nest.
I've been keeping a few hives for the past 20-25 years, and no longer treatment free as I got tired of the losses.