Respecting @DA01 mention of H-clips, an addition often dispensed with by skillfull sheathing installers (by the trick of the trade by which uniform gapping is achieved without clips):
At first I thought the sheet bowing was being dubiously attributed to lack of "edge confinement" by clips, their primary purpose being related to sheet expansion allowance. However, looking at where H-clips could have been, it also seems evident that there is no expansion gap between sheets--they seem to have been nailed down in snug fit by the original, very-likely-unlicensed, roof and shingle installer: the sheather installed panels wrong; the shingler failed to insist on correcton before felt and shingles went on.
After many (or even few) years of seasonal sheathing heat expansion, accompanied by sheet bowing and nail pulling/loosening, it would be no surprise to find loose and softened panel undulations more or less paralleling the rafters; and a good likelihood of water intrusion if shingles and paper tearing occurred...leading up to the need for (what can only be best described as stop-gap) repairs.
Since shingle roof repair is usually more difficult and more problematic than new roof, the likelihood of any recouping on account of the recent repair defects is slim to none--especially to unsheath and resheath an entire roof originally sheathed by another "roofer."
It would be a safe bet that most licensed roofers would spot such a defect and decline to do repairs--something which which roofers generally avoid anyway--for fear of "buying" a householder's existing problem or impairing his own license and bond.