Read receipts on iMessage when the person deletes the conversation

Hi,

I recently noticed that when I have a conversation on iMessage with a person that has the read receipt turned on, and that this friend has read my messages at a certain time, the "read at (time)" disappears and is replaced by "delivered" after a certain amount of time, when I close iMessage on my Macbook. Then, if I send a new message to the person, the new message will have "delivered" whereas the old one (who used to display the read at that changed to delivered) is displaying the "read at" stamp again. My theory is that the person deletes the conversation so it deletes the "read at" time stamp at the same time. I've noticed the same thing on my iPhone but now it only does this on my Macbook, whereas the "read at" time stamp is still there on my iPhone.

Has someone ever noticed this as well or has an explanation?