I picked up a second Voyager ROTB Optimus Prime today, and thankfully the legs are able to successfully tab into place. So it seems that the one I grabbed in Singapore was a dud. Although the one that I got today has one fuel tank pops out really easily and won't lock out in place, but I've swapped it with the same part from my first Optimus Prime. So now I've got where everything fits into place okay, and another with dodgy parts.