1 to 54 means just that. to force spawn it at level 50 or 54 you need to add a npc that is only available at level 50 or above. other wise if you are level 14 the enemies in the mision will spawn as level 14's
Search through the threads and you'll see a couple that address this and explain in a bit of detail how to do it right. They're not very old either.
I made sure I had nothing but 1-54 minions lieutenants and bosses but I still get a 1-14 mission.
What am I missing?