The Koranyon Propylon Chamber Key obtained inside the stronghold does not open the door to "Koranyon, Propylon Chamber" It instead opens a miscellaneous door inside the lower level of the stronghold. It specifically has "Propylon Chamber" in the key name.

One thing that might be worth noting, both the Propylon Chamber door and the miscellaneous door the key actually opens have the same name when selected in the console: in_strong_vaultdoor00