Hey there

I think we've clarified this already on discord in bug-reports 😉

Cheers, Strangiii

For others who will check this, this information was incorrect, this mission is not retroactive, and must be completed when it acatully becomes available.

From what it sounds like to me, the mission checks the current status of all secret rooms as of now, regardless of whether it is the current active mission or not.  In other words, when you open the mission section it will check the doom tower rooms and be either complete or incomplete, depending on whether the secret rooms are done.

Essentially, it means it can be done ahead of time and therefore be complete straight away once you get there as long as doom tower doesn't reset beforehand, but if doom tower does reset then the current completed secret room count is back to 0 and the mission is no longer complete.  I guess the only issue is that once it is complete the game doesn't flag it as done and no longer needing to check DT status.

As a result, it is kind of retroactive, but kind of not.  

This is exactly the case.

i