Congenital deafness is not well understood in rabbits. It could be tied to certain color/pattern genes, but there haven’t been enough studies on it. They know a lot more about the relationship between color/pattern and deafness in cats and dogs. This is an interesting article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4672198/
It mentions the broken gene (which is linked to Megacolon), but they haven’t confirmed a link to deafness yet. Coincidentally, I have a deaf rabbit and he is broken pattern. He is also a lop eared rabbit… so there’s that as well.
Why this baby bunny is struggling and appears to be deaf could be the result of something genetic. Or it could be a sign of disease. Mothers often pass things on to their kits. Sometimes, their body can respond differently. The others may appear healthy, while one is sickly. Some can become blind or deaf. One example that comes to mind is syphilis, which can cause deafness in human babies. But another disease is possible as well. It’s hard knowing since you found these bunnies outside. Whatever the cause may be, I do agree that it would be a good idea to have a vet take a look.