Longview free tier only displaying 30 minutes data

I recently installed Longview, but I'm running into this issue whenever I access it. In case it's relevant, I installed manually, as Serverpilot was already active on the Linode, following info from another user on this forum.

I've set the drop down control near top right to "Past 12 hours" and it displays that setting on each tab the control appears on. From the docs (https://www.linode.com/docs/platform/longview/longview) it should default to 12 hours.

However, the graphs only ever show 30 minutes of data.

Below the drop down, "No data for the selected time window" is displayed.

Even if I leave the page open for 20 minutes or so and then refresh the page, it will still only show 30 minutes, even though I know it was reporting data from before that time slot.

I installed Longview at least a week ago and have revisited it several times thinking I must be overlooking something.

Can anyone explain why I can't see 12 hours of data?

Thanks