Possibly...
The stock headlight runs through the starter switch...so it shuts off when the starter is actually turning.
If the light bar was just wired to the headlight, that's probably too much current for the starter switch and will eventually melt it or make it not work.
If they used a relay, TRIGGERED from the headlight but powered from the battery, then it's fine, but would still dim when the starter button is pushed as the relay would cut out.
So, check how they wired it, correct as needed.
If it were my bike, I would be doing exactly what Daniel M. said. Make certain that circuit is isolated and a relay is in it. Nice looking light bar! Don't let it get to be a problem. FWIW