I wonder if your rabbit actually has ringworm, not mites. Revolution has slight anti-ringworm properties (according to a light Google-search), so it makes sense that it may reduce the spread when you’re using it, but once you stop, the ringworm was never fully eradicated, so it comes up again.
Has your vet considered ringworm?
Medications to treat ringworm include:
General soap to clean the area prior to applying topical medications – chlorhexidine (“Nolvasan”)
Topical cream – silver sulfadiazine (“Silvadine”)
Topical liquid (like eye drops, but for skin) – clotrimazole
Oral medication – itraconazole
— Also, does your rabbit have black fur in the area of dark skin? Black fur growing in can look like darkened skin. If that’s the case, this also aligns with ringworm, because the fur grows back in areas where the fungal infection has left.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.