SQL Server Transaction log Percent Used in Perfmon
My suspicion is that your database, while showing it's in FULL recovery mode, is actually in pseudo-simple mode where the database still behaves like it is still in SIMPLE recovery mode until a full database backup is taken. 

Paul Randal, CEO of SQLSkills.com, wrote a blog post about this recovery model called pseudo-simple. This is a behavior of the database that, while it may be configured to be in FULL recovery model, still behaves as if it was still in SIMPLE recovery model. You can easily test this by creating a database and setting it to FULL recovery model. While you can perform a lot of transactions in the database, the transaction log will get truncated, as it is with databases in SIMPLE recovery model.

Paul wrote a TSQL function that can determine if a database is truly in FULL recovery.  Here's the script from the above post.

USE [msdb];
GO

IF EXISTS (SELECT * FROM sys.objects WHERE [name] = N'SQLskillsIsReallyInFullRecovery')
    DROP FUNCTION [SQLskillsIsReallyInFullRecovery];
GO

CREATE FUNCTION [SQLskillsIsReallyInFullRecovery] (
    @DBName sysname)
RETURNS BIT
AS
BEGIN
    DECLARE @IsReallyFull BIT;
    DECLARE @LastLogBackupLSN NUMERIC (25,0);
    DECLARE @RecoveryModel TINYINT;

    SELECT
        @LastLogBackupLSN = [last_log_backup_lsn]
    FROM
        sys.database_recovery_status
    WHERE
        [database_id] = DB_ID (@DBName);

    SELECT
        @RecoveryModel = [recovery_model]
    FROM
        sys.databases
    WHERE
        [database_id] = DB_ID (@DBName);

    IF (@RecoveryModel = 1 AND @LastLogBackupLSN IS NOT NULL)
        SELECT @IsReallyFull = 1
    ELSE
        SELECT @IsReallyFull = 0;

    RETURN (@IsReallyFull);
END;
GO


And here is an example of utilizing that function (replacing 'test' with your database name).

select MSDB.DBO.SQLskillsIsReallyInFullRecovery ('test')


There is also a Powershell script written by Edwin Sarmiento that provides the same information:

Check If a SQL Server Database Is In Pseudo-Simple Recovery Model Using Windows PowerShell

My suggestion would be to take a FULL backup of the database and retry your tests.