I've found a way that the mansion exploration activity could be marked as acquired without being acquired - could you share your save? I'd like to check it.
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.
I've modified the save and uploaded it here - https://mega.nz/file/uu4UlRqL#HJKzdA8afQemYj_GFD-e1aRWP3MRbjIeY0pn-FabhMg
It should be possible to find the Explore the Mansion activity now, let me know if it works :)