Dog - returning aural hematoma with no infection

Not sure what to really do about this one, dog had an aural hematoma, the vets have done surgery on him twice. The first time stitches / drainage wasnt in properly and after a week it filled back up again. The second time it healed nicely. At the same time they checked him for mites / signs of infection / foreign material in the ear / what the cause could have been.

It's about two months later and we noticed just the very tip of his ear starting to inflate, but it doesnt seem to be bothering him right now.

Anything else that could be causing this that I should look into? He's a swiss shepherd