When formatting a script that contains STRING_SPLIT with the option "Add/remove square brackets" set to "Add brackets to all identifiers", the first unbracketed instance of STRING_SPLIT gets brackets added. Subsequent instances within the script are left unbracketed (the expected behaviour).
Using SSMS 18.1 (SQL Server Management Studio 15.0.18131.0)
SQL Prompt 9.5.8.10521
Using SSMS 18.1 (SQL Server Management Studio 15.0.18131.0)
SQL Prompt 9.5.8.10521