So if you had unlocked the door to Sarkomand, it worked, and if you re-entered the dungeon, it would remain unlocked.
However,
there is a way in this game to "re-form" the dungeons, and if that happened, then the door would again be locked. With the key already found, and taken from your inventory, there was no longer a way to unlock it!
So now the game will remember if you had unlocked it, and it should remain unlocked.
However...
If all of this happened before this update, then I'm afraid that door will forever locked.
However!
If you really me want to, I can explain how to modify your save game file so it will unlock again.