Really? That's weird. Maybe it was really popular.
I think I remember seeing on BBTS that Optimus repacked in one of the next two waves of new figures, so could be this one.

I just looked at BBTS, and Wave 4 has Optimus repacked into it, but not sure about Wave 3 as they only offer sets. (their wave numbering is 2 out)

Okay... looking at Entertainment Earth site (can't be bothered retyping the post with each update ), Wave 4 does indeed have Optimus, but Wave 3 that Ultra Marginal found does NOT. (their wave number is out by one, count Stinger as wave 3 instead of 2.5 or 2R1)
Saying that, the Stinger wave has Optimus as well, but that wave seems to be up in the air at the moment, as the next wave (that UltraMarginal found) has now showed up... and very few sightings have appeared globally as it might have been intended as a Walmart exclusive (it started showing up in US Target, WITH Walmart exclusive stickers on them).

So good news for those in Singapore wanting Optimus, but still a bit of a wait if you don't get the Stinger wave... and less of a chance of seeing it because it is the last wave of a Movie line. (often last waves see limited release or cancelled)
If you find Lockdown in stores, two of those revision waves had Optimus as well according to Entertainment Earth.