We have a mu-plugin that defines some constant and our theme uses the "after_setup_theme" hook, which in turn uses constant defined from our mu-plugin.
However, this does not work as the constants defined in our mu-plugin are not available at the "after_setup_thme" hook. Switching the action to run on "init" instead fixes this issue. I am not sure if this is a bug or is the intended behavior. I was under the impression that mu-plugins were loaded before everything else.