I'm replaying the game on new game plus, and I got to the mines.
There's a purple key in the lower right of the darkest room.
Later in the level there's a wall of purple doors.
I use the key to open one and then get killed by a rockman before I can reach a portal.
I respawn in the overworld, and the purple key is gone, and all the doors are relocked.
I can't even take the sewer passage because the newly expanded ravine blocks the passage North.
I've tried quitting and restarting, and going into different areas of the overworld to hopefully reset the purple key, but to no effect.
It appears I am stuck.
How do I continue?