To be honest, before Indriyati Sekar, alias the queen, realized the reason why her transparent wall was starting to thin out and only saw her own wall slowly starting to corrode, the woman immediately decided to make another special rule.
One of the rules is that any wrena who carelessly enters Earth without fulfilling the pre-existing conditions will not be able to return to Yamani.
They could have stayed on earth as ghosts or demons, or they could have even stayed on earth as fellow humans and given up their habit of eating humans. No one realized this until the wrens began to enter the earth and cause havoc; only then did the Candrasas begin to notice these strange things.
The queen regretted that even all of Candrasa could not see this sooner than she expected. They only saw it when there was chaos going on, and it was too late for the Queen.