It would depend on the substrate and the background, is either darker in the 20L?
Is there any possibility that the driver could be damaged? Are any of the LEDs brighter than others? If it's still under warranty (IDK if they do that in this industry) send it in to get replaced and forget about the issue. If not then I'm sure we can figure it out.
About how long has it been running (like a rough estimate of the hours)? How many LEDs are there on the board and what wattage is the fixture?
People are too often merely stating an assumed truth from flawed observation.