In the biggest change the platform has seen since the switch to 280 characters, Twitter has changed the way it counts characters for emoji. Previously, some emoji — like those with additionally defined gender or skin tone — counted as more characters than others, due to the method of their encoding. But as of now, all emoji will be counted equally as two characters.
According to the Twitter Developers announcement post, this change is a result of recent updates to Unicode, which presumably allows for the affected emoji to be encoded more efficiently. Twitter's twitter-text Open Source library has also been updated with this change, so developers can take advantage of it in their own apps or services which might be parsing tweets.
— Ryne Hager (@RyneHager) October 12, 2018
Now you can be confident that you'll get every single emoji out without resorting to a tweet thread.