Interior Trim Joint Keeps Cracking: Caulk, Refasten, or Rebuild?

I’m trying to diagnose a failing joint on painted pine interior casing around a replacement window. The joint is a mitered top corner, and a hairline crack has opened through the paint and caulk. It is widest near the outside corner, while the rest of the casing feels solid. I’ve noticed it becomes more visible during the colder, drier months and nearly closes in humid weather.

There are a few finish nails near the joint, but I cannot tell whether they are still holding the casing tightly. I also do not see staining or obvious moisture, although the joint is close to the window jamb. The existing caulk is brittle and separating from one side.

Would you simply remove the loose caulk, recaulk, and touch up the paint, or is this a sign that the miter needs to be refastened or rebuilt? I’d like to avoid driving nails into anything near the replacement window. For those who have repaired this kind of seasonal joint movement, what signs helped you distinguish normal wood movement from a loose casing or moisture problem?