One of the "strings" of LEDs has died.
LEDs are wired in "series" sort of like a string of Christmas tree lights, one fails and they all go out in that string.
Often in marker lights the manufacturer will use two series strings, when one "string" goes the light will be half as bright.
Generally nothing you can do to fix these as the fixtures are typically sealed and or the circuit board has been sealed in a water resistant coating.
Yeah, these LEDs are supposed to have 50K-100K hrs of "life", PROVIDED they are not overdriven. Sadly, most manufacturers are cheap and tend to overdrive the LEDs to maximize the brightness instead of life, so far half of mine have died.
Fortunately when I bought my LED fixtures they were exact fit for the older incadescent lights that they replaced..
And my good fortune was they didn't seal the lens to the fixture which allows me to replace the offending dead LEDs with BETTER designed LEDs which most likely will outlast me.
FOUND HEREThose LEDs are designed for industrial outdoor signage and rated 3/4W at 12V which is pretty bright so I added in an additional resistor which cut the current and brightness in half. They just barely fit my oval fixtures..
The "fix" for yours is to replace said bad fixture..
