If you still have a savegame from right before your son comes of age, so that the devs can see the bug unfolding, that would be helpful.
Are those random courtiers all of the same dynasty as the ruler?
I ask, because there's another bug regarding that and I think this one could be related.
Ie, my guess is that the AI isn't actually generating random courtiers specifically to give stuff away. Rather, it generates a random courtier for some other reason. The dynasty-bug makes that courtier be of the rulers dynasty (even though that shouldn't be the case) and the AI thus gives him a county to stop the decadence gain. Now, it generates a new random courtier to use him for whatever reason it needed one in the first place and the whole thing repeats until the AI runs out of counties to give away.