I started a new save of the game and set landfill cost to 2, making it 20x cheaper, because I didn't want to have to deal too much with that logistical challenge of the mod. However, I've unlocked the Voidfill research and it takes 28s to manufacture, which isn't proportional to the decreased cost of the landfill recipe by stone. Is this a bug, or is it working as intended?

It's working as intended in the sense that the formula is spitting out the values that it should, but it doesn't make a lot of sense for very low landfill costs. I changed it so that if the landfill cost is below 20, the voidcrafting time is just double that, so in your case it'll now take a much more reasonable 4 seconds.