I agree that if it is performing like this it either is a bad batch or it is not "Sunbrella"--another possibility is that you have aluminum bows without or bad annodization, and that salt water sat on the bows, causing detiorration of the Sunbrella . Or some acid got on the canvas.
Unfortunately there are copies of Sunbrella from SE Asia and they are not satisfactory for boat use. Sunbrella is made by Glen Raven mills. Glen Raven does make some lighter fabric, but it is not Sunbrella. We have used Sunbrella for over 30 years and never had a problem, even with full time cruising in very severe elements. Sunbrella is plain, Sunbrella Plus, which has a coating on the inside which does make it fully water proof. and SeaMark--which is Sunbrella with Hartz vinly coating on the underside.
Agree contact who made the canvas, check out the actual fabric--and please post some photos of the areas. This should be made right!