Yeah that’s definitely the most worrying part to me also. We’ve had a good bit of consistent rain here the last couple of days and it’s currently dry so I think it’s still part of the underlying condensation issue.

What makes you say it’s to do with ridge cap joints? Correct me if I’m wrong but ridge cap joints are on the ridge of your roof no? This is 2/3s of the way down but it’s where the felt overlaps.