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I ran into the same issue, but was able to use the following workaround:

  • Return everyone to mansion (may require harvesting all resources first?)
  • Save Game
  • Reload Game (may require quitting and restarting?)
  • Return to completed dungeons
  • "Forget Dungeon" Button now appears like it should

I think that the game expects you to kill the boss with one team, and harvest with another -- it's the most efficient way, if you have the manpower and the gear for it. When you do it in one visit (with one team), as we all do in the early game, you reach this "oops! didn't think of that" state -- so no Forget Dungeon button for you :) .