Don't quote me on this as the following are my own experiences and not gospel:

I don't think Air Asia bothers with actually issuing an e-ticket number. They don't have any concept around interline so everything is just closed-loop within their own res system. So could be only PNRs are used and that is sufficient for them.

Also if you book via OTA there is a good chance you cannot view your booking online (hit and miss depends on how the OTA did the booking) but you can always see the booking in the check-in section.