I recently helped a friend with this issue. Below is the "fix":
Navigate to Settings > Mail > Accounts > Tap on the account in question (Yahoo, Gmail, etc.) > switch on Calendar
This should instantly re-synch calendar events to your Apple Calendar app and you should be able to search for past and future events again. It is worth noting that when you create an Event via Apple Calendar, pay attention to which account you are actually saving it to. For example, if your default account is Yahoo, you're actually saving it to Yahoo calendar while Apple Calendar is just a proxy.
Hope this helps!
-Cesar (Former Apple 5th Ave Tech Specialist)