Louis has a list a mile long of dogs that he doesn't like because they frighten him. The worst offenders are big, bouncy dogs that have no manners and come charging up to him. Where I live that's typically Labradoodles. I'm sure there must be a local Labradoodle breeder who specialises in breeding really hyper dogs and then selling them to numpties
. Anyway, rant over because I don't like to be breed or crossbreed specific, that's just my experience with dogs local to me.
Louis is also afraid of groups of dogs, so I don't let him meet them. There are a lot of dog walkers who walk where I do, plus groups of friends who meet up and walk their dogs together. I used to be able to do that when I only had William, but Louis can't cope so I don't force it on him. I've used the LAT (look at that) technique to help him have the confidence to stay calm if a group of dogs passes us nearby and I've accepted that's probably as good as it's going to get for him.
It sounds like Murphy is fearful of certain breeds and groups of dogs. The difference in reactions might be that he has enough confidence to warn off a single dog but is so afraid of groups of dogs that he starts to shut down. If he was mine I'd avoid any close contact with dogs that he's fearful of and keep him at what he feels is a safe distance. When he's at that safe distance you can reward every positive behaviour he shows, even if it's just a glance or managing not to bark, with praise and a treat. That should help him learn that he can stay calm. If it works as well with Murphy as it has with Louis you should find that Murphy's confidence increases and his safe distance becomes less, so you'll be able to be closer to the dogs without him getting worked up. Maybe in time he'll be able to cope with meeting those particular dogs, but don't push him. It has to be done at Murphy's pace, and if there are still some dogs that he can't cope with just accept that, avoid them and let him interact with the dogs he's comfortable with.