Dog only chases cat when roommate/owner is home?

Hello everyone, I'm looking for help understanding my roommate's dog's behavior so we can treat it better.

Dog is 5yo hound mutt. He hyperfocuses and chases my cat. He's made great progress with using the "look at me" command and high-value treats, but I've noticed his "lock in" and chase only happens when my roommate, his owner, is home.

I get home from work first, and will put the dog on a leash and take him into my/the cat's room. He doesnt care about the cat, he just wants pets. I sit on the floor and give both animals treats, the dog with the look at me command, but I hardly need to. They've sniffed noses and hung out with zero issues.

When my roommate is home, the dog is on alert. He watches the stairs for the cat to come down. He'll chase, though he's getting better every day with stopping or holding himself back since we started training every day. We train by having me hold the leash and practice the look at me command with treats while my roommate goes into my room to pet and love on the cat (with breaks to give the dog love and treats too).

We noticed this extends to other dogs as well. My parents dog sit him, and when my roommate is there, the dog will lock in and stare at a smaller dog, occationally going up to it to bark at its face. When my roommate leaves? His dog barely even looks at the other one. Not a care.

So whats going on? We think its protection, but how to we address it? Does anyone have resources we can read, or what we may need to look for in a trainer?