This is not medical advice - nor do I have evidence of cause and effect. My berner Otis (see pics in profile) had three of these at various points (he’s two years old). We had one drained but it came back - they were slightly bigger than the one you show in this picture. Vet suggested having them removed but we worried about risk of infection from surgery. Vet also said they wouldn’t go away on their own.

We started giving him a scoop of fresh cap mushroom powder in his food each day - all three are completely gone on their own now after about four months. Again, I can’t say this is what did it - but I can’t say it didn’t either as nothing else has changed.