This is the expected behavior for the astro items. The item has a DateTime state that contains the precise time the event will occur that day. At midnight each day all the astro items will change to the correct time for the new day. Astro items do not change at the time of the event.
This is the correct way to go, and you’ll note it actually doesn’t even use the item, it uses a trigger channel instead (see here). So the actual question is why didn’t the rule do what you were expecting. You’ll have to show us the full rule for us to find out.