xp_restore_headeronly on a TranLog backup file returns a single-column entitled, IsCopyOnly with no value.
If you run the same xp_restore_headeronly on the next TranLog backup in the sequence, the normal columns are returned.
The contents of the TranLog backup file can be read using LiteSpeed Log Reader, no file corruption is evident. There is a gap in LSN sequencing between the previous TranLog Backup and the subsequent TranLog backup meaning that some LSN is contained within this TranLog backup (corroborated by LiteSpeed Log Reader).
During Tranlog backup, Engine is not getting the correct backupsetid & mediasetid. It then fails to create a proper backup header. As a result, the fileheader XML is missing.
Enhancement Request CR70314 created so an error message is logged should Litespeed fail to write backup file header.