How common is yellow mucus in shiba puppy's eyes?

Our shiba puppy is about 4.5 months, and his eyes have us confused. Initially, one eye was swollen shut. Went to the vet, they couldn't figure it out and told us to monitor it. A few days later the swelling went away, but yellow mucus started to form at the end of the eye. Now, his other eye is creating the same mucus. We've been to the vet 3 times and they say it's fine, but is it?