Some rabbits genetically can have thinner fur at the back of the ears like that. Possibly even more so in hotter months as they use their ears to help cool their body temp.
As long as there isn't excessive scratching, flaky skin, lesions, or debris in the ear, it could just be normal for your rabbit. I had a lop that had thinner fur on the back of her ears and it was just a normal thing for her.
Though if you are ever in doubt there could be an underlying health issue going on, it's always best to have an experienced rabbit vet check it out.