How to simulate a situation when non-empty Transaction Log file will be added to the SQL Server Full Database backup file
There are two things going on here:
1: The database is effectively in simple recovery mode when doing your modifications, since you never did your first full backup after creating the database.
2: The column BackupSizeInBytes seems to be 0 for all ldf files, quite simply, even when there is log records in there. The documentation is vague for this column, so it difficult to say whether it is by design or not.