Hey, no, it's not a bug. The last heart is based on your performance in the investigation. If you correctly guessed at least half of the elements of it before arriving at the criminals' hideout (culprits, main target, objective, mole, the deducible elements in the mine if you visited it, and finally the symbol), then Serenity will compliment you on your performance and you'll earn the point (with slightly different dialogue if you got everything right).