Update on validity of registration, it seems like the use of auto gates is linked to the expiry date of the visa you have when you do your registration.

I tried entering using the auto gates a few days back, just after the expiry date shown on my previous visa (but having a new and valid visa), and the gates wouldn't open for me. Same thing happened to a friend under the same circumstances. I'll go about doing the registration again next time I come back from some trip overseas.