It says in the rules that a mini-scene only lasts until the end of the turn of the person who created it.
What if he's interrupted?
In truth, I've never had a Mini Scene Interrupted myself. By their very nature they tend to be little quick cuts that color in some pieces but generally don't need to get interrupted.
By technical application of the rule: when the player gets interrupted...his turn is over...therefore the mini scene is over.
Therefore the Interrupting player is now playing back in the main scene.
Of course there's nothing stopping either player from starting a "new" mini scene that picks up where the interrupted one left off.