Mannequin in sleeping pose is perpendicular to the rotation

When you summon a mannequin, it is (by default) facing south. Change that mannequin to the sleeping pose, and it will lie east/west, facing east. It should lie facing south also.

To reproduce, summon a mannequin, then run:

/data modify entity @n[type=mannequin] pose set value sleeping

You will see the mannequin lie down as described.