Biodiesel is currently required to make cooking oil, and Snacks require Cooking Oil. However, it's still possible to research Snacks without having researched Biodiesel, which is pointless.
I would propose either:
Just noticed the same on a run where I rushed some advanced tech (specifically exhaust filtration) and am now filling through, trying to get a switch to Hydrogen working early.
IMO, since the mill is already unlocked with Wheat, I'd just add the ability to produce the cooking oil with the unlock of its corresponding raw material. You may not immediately have a use for it yet, but it'd allow you to produce the goods for it and maybe start stockpiling the oil for later use.
which I guess just means shifting the soybean -> cooking oil recipe to the soybean unlock, as Canola unlocks it as suggested already.
You still have a good case for a hard lock on Snacks to depend on at least having soybeans unlocked. Or is there a way to get cooking oil through trade, and this would basically be a similar behavior to how Glass used to behave for the tier 2 vehicles, in that you could only get it through trade for a while before unlocking it to manufacture yourself?