I think this is normal behaviour. Computer controlled Ships (and also player ships wet to auto attack if I remember correctly) always target the first ship that jumps in. They only switch targets if a better target jumps in, i.e. a ship that can be damged more effectively with the ship's weapons then the current target. As cruisers and LF are strong against scouts, they have no need to switch targerts. An exception are colony ships, they get always attacked with highest priority by the AI ships on neutral planets. Knowing this you can split AI forces to take them out more effectively.