Legacy Type Movement: More freedom to view your character while moving

[Rongway]

Exactly, why does left mouse work like it should while auto running, but not during normal running?
It seems like such an easy / obvious thing to fix

It's the way the movement buttons work, not the mouse. The mouse does what it should do: move the camera. That much is working correctly. The thing about movement, however, is that legacy mode is by definition camera-based movement. So when you press the movement keys, your character will always move the direction you press, oriented with the camera. That much is also working correctly. When you stop pressing the movement keys, you can move the camera without changing your facing again.