Passive mobs don't spawn if you're riding a horse

Passive animals should spawn in some newly generated chunks if the biome supports it, but I have found that if you're riding a horse then no passive mobs will spawn in newly generated chunks. This was tested (and very apparent) in a superflat world, where I explored a lot on horses.

Once I figured out the cause I started running on foot and sure enough, passive mobs began popping up on the newly generated chunks. This may occur whilst riding pigs or boats - might want to check the code for those. The logic might apply to minecarts, but obviously if there's rail there it's not newly generated chunks.