How do you manage someone who used to be your peer without resentment?

A few months ago I got promoted to manager over my small team. The thing is, three of my direct reports were my equals before this. We’d grab lunch together, complain about the same stuff, and vent about the old boss. Now I’m the one giving feedback and approving their PTO. Most of them have been supportive, but one person in particular seems genuinely resentful. They’re not insubordinate, just distant. They stopped coming to team lunches, push back on small requests with a “well you used to think that was dumb too” tone, and I overheard them tell someone “it’s weird now.” I’ve tried having a casual check-in and said I miss our old dynamic too, but that didn’t really change anything. I don’t want to pull rank unnecessarily and make it worse. But I also can’t manage out of guilt or pretend the old dynamic still works.

For those who’ve been through this, how long did it take for things to settle? Did you have a direct conversation about the elephant in the room, or just let time do its thing? Any specific language that helped without sounding like you were lecturing an old friend?