This is a direct result of E2EE (End-to-End Encryption) protecting your privacy.
Local Storage: To ensure maximum security, your chat history and encryption keys exist only locally on the device where the messages were sent or received.
No Cloud Risk: We do not back up your plaintext records to the cloud. Therefore, when you log in on a new device, historical records cannot be synchronized because the new device lacks the local data from the old one.
Privacy Assurance: This mechanism is designed to eliminate the risk of cloud data breaches, ensuring that your conversations remain accessible only to the actual participants.