I had, if anything, the opposite problem. I got awarded the milestone just for building a farm, despite it being non-operational as I hadn't any colonists yet.

Given that it takes so long to gain the open farm tech, and to set the terraforming conditions to allow it, are you sure that one of the AI rivals did not pick up the Produce Food milestone before you got your open farm producing?

Were you by any chance producing the default Cover Crops only? I don't think those produce any food, just seeds.

Were you playing with no rivals?

@Intyalle:
I got nuthin' on yours, ma'am. Just a glitch, I think. I have completed that situation (farm built, no colonists down) probably 25 times, and don't recall ever getting fireworks for it.

I played without rivals, so they aren't bothering me :) I was producing food with it, before that I only had import, so when it started producing, it was both fast and quite visible. My terraforming was was way high, so no problems there. It just didn't register as "food production", probably just like how importing doesn't register either.