Thanks, looks like what I thought may have happened has happened.
There's a 'discoveredMansion' value near the start of save files; this can be swapped to false so the Explore the Mansion activity will generate again. The same applies to 'discoveredRealmOfHeroes' if the Realm of Heroes is missing.
If you'd like, I can do the edit for you; I'll just need to know which save.