Reddit is moving to a new chat architecture, removing all the chats and messages before 2023, as the company believes pulling older messages will enable a "smooth and quick transition" to the new architecture. The change took effect on June 30th, but many users only noticed days later.

source https://www.gadgetsnow.com/social/reddit-is-deleting-old-chats-heres-how-you-can-get-them-back/articleshow/101788295.cms