More cracked shingles here.
House is 1960, roof framing is 2 x 6 16" OC with plywood sheathing. These shingles are a second layer and are about 15 years old. I encountered 6 or 7 vertical rows where the cracking went through 23 courses, 3 cracks were about 6’ apart from each other and other areas had random spider cracking.
Would this be due to the bottom course, there were no issues of structural failure from within the attic space. It had good Gable venting but the soffit venting did not appear to be as good, with no lights on in the attic, there was not much daylight visible from the soffit vent areas.