I got a response back from the help desk as follows:
On escalating this to our technical team, there has been an issue identified, which there isn’t a fix for at this time, but they are working on it!
However, there is a workaround in the meantime. If you set-up your account to receive weekly statements, once you’ve received these then you can revert back to monthly. Strangely, setting statement delivery to a different timeframe can often solve the issue.
Lastly, after trying these steps, if we still don’t have any joy, unfortunately the only way to retrieve your journey history will be to manually download it until a fix can be created. This is very easy to do and only takes a few seconds.
I’ve tried setting mine to weekly so we’ll see if it sorts the issue. I did try turning it off and on to no effect some time back so I’m not that optimistic.