I had that enabled so it might not be the same issue that I had but here’s something I noticed when I was checking g to see if I had that Siri option enabled.
To test this I needed to remember something that old to search for in my calendar. But couldn’t think of anything to search for older than a year or more so I went to the calendar and scrolled back two years and at first there was nothing in my calendar (no dots on days to indicate there’s an event there when in year view). But then events started showing up in the calendar as I assume it started pulling events down from iCloud.
So here’s my guess, when searching originally I think the event I was looking for was older and not synced down. But if I scroll back a couple years it triggers it to pull down events and now it will find events in those years.
Either way it seems very odd that the app can’t pull search results from iCloud fast enough.