I agree that it looks as if the buckling is at sheathing butt joints at rafters.
Roof plywood buckling.
They are easy to work with readily available and perform the job well.
But they must be installed correctly.
Your roof decking is the plywood sheets that cover your home.
If the adhesive side is not laid properly and not applied flat it will surely show its buckling through the roof shingles.
A roof over or re roof is not recommended because it will certainly lead to buckling.
Check the buckling pattern for spacing at 4 or 8 ft intervals or a multiple of that.
Wood expands and contracts so there may not have been adequate planned spacing.
Buckling can occur if the wrong type of nails were used to connect the sheathing to the rafters.
Improper installation of roof sheathing panels can result in warping or buckling of asphalt shingles.
The nails slip out of the sheathing and create small visible bumps on the surface of your roof.
Buckling roof plywood below asphalt shingles is a result of many things.
For a good reason.
Plywood and osb roof sheathing are the most popular roof sheathing materials in the u s.