I've recently had this same problem happening to me. For years, I could drag a png out of the browser and paste it in no issue, but only tonight have I now had the problem of dragging it in making all the supposed to be transparent areas black.
Copying and pasting retains the transparency, but at the same token, that's a few extra steps as opposed to the easy drag that was able to happen before. Especially when I never even really touched my settings, the fact that this all of a sudden has changed is a bit irksome. The fact that it only happened to me tonight, and the same happened to you in the same time frame, tells me that there's probably something up with the program right now.