Doc's right. Your issue was the vinyl not the 303. We've used and recommended 303 for over 20 years. The 303 didn't change, the vinyl did.
In 2008, Sundance used Ideal Spa Covers primarily for their covers. Ideal was using a fabric sold by RKB Morbern that was sourced in mainland China (for years LG made SpaHyde for Morbern and it was fine). They had a problem with quite a bit of that vinyl. The tan was turning black while the browns and rust colors were fading badly. Sundance has switched half of its production to another LA-based manufacturer which is even worse because they literally buy the cheapest vinyl they can find. IMHO you need to source fabric that has a track record. My guess is you just got a bad batch of vinyl with your OEM cover.
These days with the economy the way it is, many of the supersize cover manufacturers are cutting pennies and fabric is an easy place to do it. We were speaking with a Mexico mfr earlier this year about their spa line that used to be 28 oz. When I questioned how light the new sample was they said they had switched to 25 oz to respond to demand for lower prices. Their largest spa cover customer is a place in Florida that will remain nameless. That facility builds the covers for many of the eBay merchants and a handful of decent size online retailers. Of course, you don't find any of these guys changing their websites to reflect the lesser weight fabric they are now selling. In some of their defense, they may not know as I doubt the cover manufacturer in question has volunteered the new information. That said, they're prices are very low and their clients know what kind of cover they are selling.
A little off track, so I apologize. The point is costs are going up but the public doesn't want to pay more. As everyone tightens their belts, fabric is the easy thing to trim from for a lot of these big companies. If it's not the weight, it's the additives. All red-based colors are very expensive to produce because it takes more dye to sustain a red-based color over time. Less dye = early fading. A way some of these mfrs have cut price is to back off on additives. Some are color-based, many are resistance-enhancers like UV resistance and chlorine and bromine resistance.
Sorry for the long answer, but the reason your 15 year spa had better covers is because they were made of better fabric. Hopefully, the manufacturer you choose for a replacement cover uses better fabric than your OEM cover. But rest assured, the 303 is not your issue.