Fetch an iPhone! I'm on an iPhone 6 with iOS 12.4.3, but I don't think this is specific to that device and iOS version (please leave a comment if there's a device + version this works on)
Tap the "Fandoms" field
The "Start typing for suggestions!" message will apear
Type a few characters, e.g. Test
Note that the message changes to "Searching..."
Because nothing happens
The "Searching..." message stays visible and is not replaced with a list of suggestions. However! If you tap into another field, such as "Rating," and then tap the "Fandoms" field again, that will trigger the suggestions to appear. Buuuut if you enter more characters, it goes right back to, "Search..." until you repeat the process.
After you type your characters, "Search..." should be replaced with a list of suggestions. The list should be updated if you enter more characters.