Dog randomly attacks one specific cat

This post is NOT about introducing my cat and dog, i read the introduction and the reactivity wikis and neither seem to apply here. They have lived together for about a year and a half, and she has progressively become more aggressive toward one of our cats, both are female. The cat has never done anything to provoke her, rather the opposite. For example the dog was standing still and the cat would try to gently head bump her to show affection, and the dog snapped and nearly killed her. The dog loves all the other cats, she will lick them and sniff them but has never shown aggression toward them. There doesnt seem to be any trigger aside from the cat being within biting distance. The cat may have a mild disability, she definitely wouldnt survive in the wild. Im not sure what to do, this feels like a very specific problem that none of the youtube videos ive searched for will explain. The dog has been wearing a muzzle in the house since the last attack, or be separated. Besides this one issue she is a fantastic dog, she wont touch her food unless we tell her she can eat, if any other dogs show aggression toward her, she immediately submits and lays down. I dont know what to do, i dont want to give up on her and give her away, but i also dont want to make her wear a muzzle for the rest of her life when inside. Please help i love this dog and this cat and i want to train her to be nice.