It has to be investigated from inside the attic space and on top the roof, call in a roofer…
I don’t see any ridge ventilation on the main roof and I do see the power attic vent. I always access the attic ventilation when I see something like this going on, if the attic has gable end vents only it probably isn’t enough, if it has a power vent is it controlled by heat only or does it have a dual control that powers up with the humidity level. If the power vent is expected to handle both attic areas it cannot, it is to low to work for the main roof deck and the two attic spaces should be vented as two separate areas (there should be a barrier between them).
Statistically the average family of 4 creates 5 gallons of water a day that goes up to the roof to get away (humidity…moisture) so if the ventilation is not correct the moisture will saturate the roof decking causing delamination of the sheathing, swelling that will cause buckling of the sheathing if the spacing between each sheet is too tight or taking up by the swelling. Worst case scenario is the underside of the decking is black!
Just an opinion, need more info and some better pics to go off of…could even be loose sheathing…not nailed when house was built.