The problem here is determining the actual cause of the damage.

The two possible causes that come to mind are high humidity in the failed areas, possibly due to air circulation or insulation issues ,or ultraviolet radiation damage prior to instalation.

The area can be patch with quick setting joint compound. It is not as water soluble as light weight mud .

I also recommend you inspect the insulation in your attic and provide your home with better are circulation possibly with a dehumidifier to prevent further damage.