At the end of the main missions for the TG, it's mentioned that you may be called to the Twilight Sepulcher occasionally defend it against invaders that would steal the Skeleton Key. Two characters now that I've played through the TG missions, with a good amount of hours, but no once has my character actually ever been called to defend the Twilight Sepulcher.

Am I missing something here or is this another bug that never got fixed by BGS?

No bug.

Either a quest that never got implemented or a hint that at some point (Not in game) Your character may be called upon, a hint that Nocturnal won't be letting go of you just yet.

No you're never called upon. The same goes for several other Daedric quests. The deity always says that one day you will be called upon. It's not a bug. Would've been cool though if you actually were called upon.