I have done a number of repairs to T-111. Exposed to full sun and heat it sometimes deteriorates as you describe. For minor repairs, I have filled and painted, but when the sheet warps or delaminates, replacement is in order. T-111 is usually nailed at 4-inch intervals on the edges and 8 to 12 inches in the field. It is easiest to cut the sheet out cutting a couple inches inside the sheet with a circular saw set to the depth of the sheet, removing the field and edges separately. It can help to cut the sheet in half. Removal is a lot of work, and it is difficult not to damage adjacent sheets, but it can be done. Pay attention to the direction of overlap as you pry the sheets off and release adjacent sheets. Install new Z flashing above the replacement.