It appears the issue is that my contacts where read receipts are not appearing, have not yet accepted the new contact request for my new email address.
It may be a bit surprising that read receipts are not sent for already read messages even after accepting the contact request. Could be interesting to check how other messengers solve this problem.
I think this problem will go away if all my contacts are verified should I need to switch email accounts again. Will update this thread with the results.