@zenid10, I think it's a transition rather than a bug :) but I see your point. Also, a bug is something that is a mistake and this is more of an intentional change that we know will take a little bit of work to switch to but we feel that the benefits in the long run will work out in everyone's favor.
Sorry it's been causing trouble for you!
I have a tool now that will let me remove all size-thumbnail, size-medium, and size-large class name from the images—doing that will disconnect the sizes on the Settings > Media page from your images so you can't use the Settings > Media options to resize already-embedded images any more and so the settings won't affect past images you embedded in the past. Does that sound like something you'd like to do?