No transparency in rendered background, is this a bug? Suddenly I only get a gray background in my cycles renders, even though film-->transparent has been checked and the format is a PNG with RGBA enabled. The image still contains the alpha information (verified in another software) but why is Blender displaying this gray background?

Here is 20 sec video demonstrating the problem:
https://youtu.be/1FqZaiTiCFU
I filed a bug report and did some testing. Seems like the cause of this problem might have to do with my graphics drivers.