what could be preventing my transaction log autogrowth to be set to UNLIMITED?
If you open BOL article sys.database_files (Transact-SQL)

you'll find that
268435456 = Log file will grow to a maximum size of 2 TB

2 Tb is the max size of SQL Server database log file, if you need it more than 2Tb you add the second log file.

Here you can find Maximum Capacity Specifications for SQL Server that states that 2Tb is the maximum log file size: