Transaction Log backup overwrites full backup on same media
You are specifying INIT on both your FULL and transaction log backups.  

If you want to 'append' the transaction log backups to the FULL backup, specify INIT on the FULL and NOINIT on the transaction log backups.

If you don't want your transaction log backups 'appended' to your FULL, I'd suggest generating a dynamic transaction log backup command 'appending' a timestamp onto the transaction log backup file name and using INIT.

Here is an 'example'

SET NOCOUNT ON 
declare @BackupTimeStamp varchar(25)
declare @BackupCommand varchar(max)
declare @VerifyCommand varchar(max)
declare @ServerName varchar(max) = (select @@SERVERNAME)

set @BackupTimeStamp = 
    right('00' + CONVERT(varchar(4),datepart(year,sysdatetime())),4) + '_' + 
    right('00' + CONVERT(varchar(2),datepart(month,sysdatetime())),2) + '_' + 
    right('00' + CONVERT(varchar(4),datepart(day,sysdatetime())),2) + '_' + 
    right('00' + CONVERT(varchar(4),datepart(hour,sysdatetime())),2) + 
    right('00' + CONVERT(varchar(4),datepart(minute,sysdatetime())),2) + 
    right('00' + CONVERT(varchar(4),datepart(second,sysdatetime())),2)

set @BackupCommand = 
'
BACKUP LOG [SCOPERATIONAL] TO  
DISK = ''\\FBPISILON01\DBBackups-SC\' + @ServerName + '\Xlogs\SCOPERATIONAL_backup_' + @BackupTimeStamp + '.bax'' ' + 
'WITH FORMAT, INIT,  NAME = ''SCOPERATIONAL_BACKUPS\SCOPERATIONAL_backup_' + @BackupTimeStamp + ''''
print @BackupCommand
exec (@BackupCommand)  --should probably be sp_executesql


Keep in mind, that EACH transaction log backup will continue to be larger UNTIL a non-COPY_ONLY backup is taken.

Additionally, should you decide to create transaction log backups that are not appended to your FULL backups, you'll need to create some sort of pruning/purging process to delete older transaction log backups.