This is an interesting question. You have several sets of LED lights connected together, one set is missing half of the lights which is the question that having that many LEDs out would stress the rest of the strands. As long as the current for the whole chain is below the burn out threshold of the LEDs then it should be good although it would be smart to monitor the strands in case more LEDs burn out. But, when in doubt you should replace the defective set or remove it.
The leads on the LEDs are copper with tin/lead solder or silver solder, the leads can oxidize and that could be the problem. You can try to remove the defective LEDs and use a pencil eraser to polish the leads to get rid of the corrosion.