Reminders added to preset dated lists not reaching iCloud

When adding a reminder from my iPhone 11 Pro Max (iOS 18.6.2) to either the "Today" or "Scheduled" list, they don't make it to my iCloud account so do not appear in the Reminders app on other Apple devices. The same behaviour occurs if I add to one of those two lists from my watch (series 9, Watch OS 11.6.1, 22U90).

If I add a reminder to any other list (including "Reminders") with a due date, they do get sent to iCloud and therefore do appear on other Apple devices. It also works properly if I do this on my watch.

Has anyone else observed this sort of behaviour? Any idea of the cause, or suggestions for a remedy?

Thanks in advance,

Chris