You're not alone with this one. A lot of people have complained about the newer printings feeling flimsy compared to earlier versions. Board game manufacturers do sometimes cut corners on card stock to reduce costs, and Splendor has had multiple reprints over the years with varying quality. The thinner cards are definitely real, and honestly it's frustrating when you're paying the same price for a product that feels cheaper.
The creasing issue is particularly annoying because it damages the cards pretty quickly during normal play. You might want to try being gentler with shuffling - maybe do more of a riffle shuffle with a smaller portion of the deck at a time rather than one big shuffle, or look into getting some card sleeves if you don't already use them. Sleeves would protect them and probably make shuffling easier too. If the cards are legitimately damaged after just six months of casual play, that's worth contacting the publisher about since it suggests a real manufacturing defect rather than just wear and tear.