Most likely a request now goes to the "new infrastructure", where only new messages are replicated. Old data is still stored on the old infrastructure, which some old frontends are still using. After the migration is over, the old chat might no longer be accessible.

So there's hope for remedy to their tech incompetence, which is also their tech incompetence…

It appears some users may be able to download all their messages, including ones prior to 2023, by making a data request at https://www.reddit.com/settings/data-request(opens in a new tab) while logged into their Reddit account. [...]

In addition, using the old version of Reddit which is still accessible at https://old.reddit.com(opens in a new tab), users may still be able to access inbox messages prior to 2023.

However, some Redditors have reported that even the data request option did not retrieve their old chats or some messages were missing.