Ticket Numbering

How is the tickets PDF numbers do incremented and is there a TAG to include in the ticket for ticket number?
My settings in the event are:
Activate Tickets PDF - Yes
Ticket Start Number - 1
Ticket Prefix - TK
But each time the attachment in the email is the same - TK00000.pdf.  I am expecting TK00001, TK00002 etc.
 

Hello

1. The ticket number is increased one by one. So it should be TK00001, TK00002, TK00003.... Look like something strange is happening on your site, please submit a support ticket so that we can check it directly from there

2. To insert ticket number, please use [TICKET_NUMBER] tag

Regards,

Tuan