Emoji search in non-iMessage apps
Bafflingly, this was the answer:

https://www.idownloadblog.com/2017/11/15/how-to-fix-predictive-emoji-not-working-iphone-ipad/

Apparently, the algorithm for recommendation for emoji replacement is per-app, which makes sense in that you speak differently in email than you do on twitter, but apparently, if you've never spun a random string of emojis together in a new app, the keyboard has to learn it from 0.  

So to fix this problem is: open your regular keyboard in the app of interest and type some words, then open your emoji keyboard and type a ton of random emojis (not even necessarily the one you want!). After you do, the recommendations will recognize that you mix English and Emoji and recommend them together, even from your English keyboard.  They will appear on the right hand side recommendation just where they do in iMessage.

(The link used Angola, not Serbia.)