iOS Calendar & Outlook - All Day events "On day of event" alert not working as expected

Using iOS Calendar (iOS 18.4.1) to display Outlook Calendars, if creating an All Day event using iOS Calendar I'm unable to select "On day of event (09:00)" as an alert time, it keeps resetting back to "None".

If I create the All Day event in Outlook on the phone, and select remind me "Day of event (09:00)", this works within Outlook, but when viewing the event in iOS Calendar it displays "On day of event (00:00)".

If I create the All Day event in Outlook on the web, and again select remind me "Day of event (09:00)", this works and is displayed in both Outlook (phone) and iOS Calendar as correct "On day of event (09:00)".

End result is that using iOS Calendar to create All Day events I'm unable to select "On day of event" as an alert, I have to use Outlook on the phone, but this creates the alert (in iOS Calendar) for midnight, or if I do want to be notified at 9AM then I need to use Outlook on the web.

Has anybody else experienced the same or is it a quirk of settings somewhere?