So, I took a look into this, and it might be a bug? Here's the animation table for when Sonic is running on the ground. The format is that each row represents the direction Sonic is tilted, and each entry in the row represents the speed. Here's animations 2 and 3 for example, the slowest animations for turning slightly left and right. The sprites used normally are facing right, but animation 2 flips them to face left instead (by setting a bit in the highest byte of a sprite pointer, ala the "|$1000000"). Although, I cannot 100% say that it really is a bug. Maybe it's just a weird choice made by the devs, maybe it's an oversight. In the end, it's just some food for thought, I suppose.