Thanks, this value wasn't defined and now it works. (I probably missed it during the upgrade)

(Searching a specific emoji still behave weirdly by returning all entities containing 1 emoji. It seems to be caused by utf8mb4_unicode_ci / utf8mb4_unicode_520_ci not sure if it is important)

I wrote a test and it passes for me: https://github.com/Elgg/Elgg/pull/12039

Give it a try and let me know if it passes for you, and which MySQL version you are using.

Also post the exact query you are using. If you are using literal SQL it might behave differently, using elgg's DB API now runs everything through PDO and sanitizes the values according to the driver. That probably has different results than just using plain sql.