so unless I'm missing something it seems that if your inventory is full and you open a box like a chest, that the item just vanishes and isn't in the area tab?
So these items should never go to the area tab, now that I think about it. Since you are using a skeleton key to open them, the heirloom item should always just replace the space that the key took up. So I fixed it to where it adds the item regardless of inventory space, knowing that the key would be removed to free up that space.
Thanks for reporting this bug!