On a 200 light set, there are five parallel strings of 40 LED bulbs. Each bulb is biased by 3 volts. If one string is dim, it means the forward current of one (or more) of the LEDs is not passing as designed.
So yes, one or more bad bulbs is an open but may not be a true open like a popped filament in an incandescent bulb.
You should be able to remove the bulbs one a time and measure using a multimeter on diode setting and find one that has high reverse voltage and and also high forward voltage. The good ones will have a distinctly lower forward (pos on anode, neg on cathode) reading. Typically on LEDs the longer lead is anode. Also, if there is flat edge on the plastic case, that's adjacent to the cathode. Finally the anode lead is typically connected to less hardware inside the colored plastic if its visible.
You can do the same thing with a nine-volt battery and two jumper clips. Use a weak battery or make one of the jumpers a 1/4W 100 ohm resistor to keep from overdriving the LEDs. I know, 40 times of doing this makes buying another set attractive. The light guys know this. When you're reinserting the bulbs, it doesn't hurt to gob a little vaseline to keep water off the steel leads which rust.