additional context (which the OP didn't reference):
https://mysqlserverteam.com/mysql-8-0-when-to-use-utf8mb3-ov...
https://mysqlserverteam.com/sushi-beer-an-introduction-of-ut...
https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8...
hint: "Exactly the same set of characters is available in utf8mb3 and ucs2."