Database T-Log is full due to 'Active_transaction'. SQL server is using new T-log file first rather than old
Why process was failing with T-log full even though T-log file size is set to 500 GB

Is Replication or CDC enabled for the database?
select name, is_published, is_cdc_enabled
from sys.databases 


Why would SQL server start using new T-log file rather than old T-Log file first?

Because the old one was full.