I heard a couple of times, after people get a new phone number, after installing WhatsApp, they see the conversation from the old owner of this phone number. How can this happen then?
If you really mean conversations of the original owner, I can't believe that's true after e2e has been enabled.
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.
Doubtful that they are seeing the old conversations, but people with the old phone numbers can send new messages to the new account with this phone number, continuing an old conversation.
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.