Whenever a player gets created using the spawn named entity packet the player faces to as well yaw and pitch 0.

Reason for this is that the client ignores the packet because the entity has not been notified as created yet.

This can either be fixed client-side by handling entity create packets before the orther entity packets.

Or server-side by sending the head rotation packet 1 ms later.

This also happends in vanilla minecraft and all you have to do to reproduce it is join and look at a player but make sure the player is not moving.