Casting issue: Mob spawning loses precision at high coordinates

While this is rather hard to notice within vanilla bounds, it's definitely there.

One simple way to reproduce this is to create a completely blank superflat world, and build a small 3x3 platform at the lowest possible point in the world. You will notice that the mobs spawn aligned to the grid as expected. However, if this 3x3 platform is built with one corner at (19999999,0,19999999) and the opposite at (20000001,0,20000001), it can be seen that mob spawning has a very clear bias to the negative side, with it aligned to the corners of blocks rather than edges. A command block was used to help indicate when mobs were spawned.