No, not a bug, a functional change. I now see it wasn't mentioned in the change log.
It was decided that get_file() should get the file. Completely. Stripping the DOCROOT from it is a rendering issue, where you need to path relative to the DOCROOT. So this functionality has been moved to the render method.
So if you do the rendering yourself, you need to strip the DOCROOT yourself. Or extend the Asset_Instance() class, and add a get_relative_file() method or something, that does this for you.