My dog barks at co-owner but not me

For context, my roommate and I have a puppy (11 months old) together that we got in March of this year. About a month ago, my roommate and I went on a trip. Four days later, we came home. When I walked into our apartment, our puppy got excited and was happy to see me (not barking) but when my roommate walked into our apartment, our puppy barked loudly at him for about half a minute. This has happened twice since then (other than we were only gone for a few hours those times). Our puppy doesn’t even behave this way towards strangers that come into our apartment. He barks at other dogs and rarely a stranger on a walk. I always walk in first and my roommate follows right behind me. My roommate has a good relationship with our puppy. They cuddle and play with each other. I do spend more time with our puppy since my roommate works full time and I am only taking online college classes.

Does anyone have any ideas why this might be happening? Or how can we address this issue? I want my roommate and our puppy to have a good relationship, but my roommate is getting anxious about the barking.