In Animation Blueprint, Update_EssentialValues function, the comment:“Caches the root bone transform from the offset root bone node if possible. Storing the offset this way instead of using the Get Socket Transform function ensures a more accurate value. Since Skeletal Meshes are rotated -90 degrees by default in editor, an offset is added to make angle comparisons against other transforms easier.” when calculate RootTransform, there is 90 degrees added. And in CBP_SandboxCharacter, the mesh rotation yaw was set -90.