Whenever I and my girlfriend talk about anything too personal, I always joke about the government's ability to hear our conversation.
WhatsApp and Viber provide cryptographic communication but, if I'm not mistaken, it needs to be explicitly enabled and I've also read there's a bunch of metadata exposed.
But it could be that messages sent to the original owner are received, since WhatsApp automatically re-encrypts and sends messages if the message has not been received yet and the key has been changed. So basically that would mean the message would be sent when the previous owner already changed their number. So the people shouldn't have sent the messages at all.
WhatsApp e2e encryption just makes sure that the only person that can read the message is the owner of the number, not necessarily the person you want to send it to.
I think this is why even simple things such as wiretapping and checking which cell phones registered near the crime scene still work.