Can Snergle's Caverns Fully Be Replayed?

[EstelRandir]

On multiple playthroughs, it seems that you cannot open the final door to Snergle's chamber with the key from Snergle's mines. It simply will not work. I have destroyed previous keys (dumping in water), allowed all zones to properly reset, gone to the mines, got the key, went to the caverns, and the door will not open again with the key. I am guessing this is a one time thing (a flag in the savegame)?

Is there some way to force Snergle's door open in the caverns without using a hex editor?