Actually, the previous thumbnails were cropped. You saw only a part of the image in its full dimensions. Now, it's the entire image scaled to a rectangle. It is possible to restore the previous view, and I will add the option for configuration.
Ernest has it correct. Thumbnails now show the full image when before it was doing of cropping and scaling and resulting in things being distorted or just straight missing.
Edit. Scratch that, there's some cropping happening that shouldn't be. A lot of time was spent to make sure images retained their original aspect ratio but it looks like something has gone screwy with the thumbnail service.