Leaking from roof between a one-story sun room and main structure

If there are signs of past water damage I would recommend opening the wall from the inside just below the leak to attempt to determine where the water is entering. Drywall repair is cheap versus the problems of moisture in the wall long term.
I just had a similar situation with damp drywall in a ceiling corner. In opening the wall there was a leak from flashing that wasn't properly installed that had been going on for years. The top double plates, the king stud along a window and the rafter base were all rotted away. It had been leaking for years without soaking through the drywall.
I would cut out just enough drywall to inspect the framing and roof area from the interior. Look at the framing members for any sign of moisture present or past. Water leaks sometime do strange things traveling along rafters, etc. before it becomes evident. As you get in there you may have to take out additional drywall as you track the leak.
Calling the roofers who did the installing is a good first step but until you determine the source any solution is only an educated guess.
Edit
As I said in comments: These leaks are usually easier to diagnose from the interior if you have possible access through drywall. If not, obviously, have the roofer check areas of the roof above the leak - even on the other side of the chimney chase. This is often a common problem area.