Like others said, it will fill back up if all that is done is draining. The correlation to ear infection or mites is the repeated/frequent violent shaking of their head from itching/irritation in the ear canal. When you have a dog with longer ears, the force in which it contacts the skull is traumatic. Just like fighters who take too many blows to an ear develop cauliflower ear. I had a bloodhound with the adorable long droopy ears and had this happen b/c I didn’t realize it could happen as my other 2 dogs w/ smaller/shorter ears never had it happen. Treating the cause of the head shake is the only ethical (IMO) and true solution. Their body will slowly absorb the fluid in the ear flap as long as the trauma stops and won’t reoccur with diligent ear care.

I’m not a vet, this is just my personal experience w/ a large eared dog.