The "other sim" must have finished the opportunity after whatever-bugged-it happens. I, for example, haven't tried to order omni seeds since before installing Late Night, so I can't be sure if it got bugged from the expansion, the patch(es) or because I moved the lot to a different neighborhood.
However, after moving my sim, I also merged 2 more sims to the household, because I figured he could use some company, and one of those sims completed the gardening opportunity-chain and is able to order omni seeds.