i’m having the exact same problem, and nothing seems to help :/ this is extremely frustrating.
I have been having the problem when I'm on Wi-Fi. If, however, I turn Wi-Fi off and just use cellular for the search, it works. I'm on an iPhone 7 (32 GB) running iOS 11.4.1
Hope you are doing great!
The issue with Search function is fixed now so it should work for you.
Let me know if you have any other questions ;)
Have a nice day :)