Edit: Restarted and all went fine this time. I think what might have happened is that during the Rocks Village quest with the heavy Rock I might have dropped all my items on Eric to hold the rock, and when I returned to get the bags didnt notice some had gone missing. I'm not 100% sure this is what happened but I think it's most likely. So it's decent lesson about not leaving items on the ground because its easily possible they'll simply despawn.

I'm playing as Eric and I've just gotten to the other side of the river and received the Water key. I've noticed somehow I seem to have lost the Moon Key.

I have no idea if I dropped it, accidently sold it to a seller or if it has bugged somehow, have I stopped myself finishing the game?

I've tried looking around but I have really no idea where it could have dropped or if it was sold. The guide/s on the game aren't very clear on whether this is a scripted thing like how the dragon amulet piece was stolen by a thief.