Many people in my area have had problems with Certain-Teed shingles. They have deteriorated after half their guaranteed life and any compensation has been very small. I too have gone with Elk Shingles. Hopefully they last much longer. Good luck.

thank you both for taking the time to respond. I'm glad to know the gap is supposed to be there. Millworkman, I don't see an end cap (see picture.) I also don't see any side caps but I suppose there are different ways to do a ridge vent. We also had that seam at the roof peak (above the window) re-caulked when the roofers checked out the roof. Could a hairline seam cause leakage? The roofers said there were copper nails and some of the caulk around the nails had worn off so he re-caulked some of them as well. One other thought, we were away for two weeks this summer and the a/c in that room was off. The room gets lots of sun. Could mold have formed in just one small spot from prolonged heat and humidity?That seems like an unlikely explanation . Right now we're waiting to see if it comes back. If it does, we will open the ceiling near it. Sorry this is all over the place, but we're just trying to find a reason for the mold since the roofer could not see anything obvious. Thanks again. I really appreciate your help.

Jim- We're in Denver, so very low humidity, lots of sun year round, never super hot, never super cold. The "breathability" of the tile vs the asphalt is also part of my concern as I figured that little bit of open space under the tiles helps keep the decking cool...

Looks like you are missing part of a ridge cap over a ridge vent. I do not think it is urgent.