Emoji search in non-iMessage apps

I have English and emoji keyboards enabled. In iMessage, I can just type "serbia" and I get the emoji for the Serbian flag. However, in Signal, when I type Serbia, I just get Serbia and Serbian as suggestions. I have another friend using the Signal app on iOS who does get emoji recommendations from his keyboard, so I have seen that it is possible, but all my recommended auto-replacements are textual. I don't understand why this is different on my iPhone X and his iPhone 7. It's very annoying to have to scroll through all the emojis and pick one rather than just type a word and get it. What is happening and how do I fix it?