Definitely a design flaw. Even though the brake/signal lights seem to be working correctly, I'd install blocking diodes in both that wire and the park light wire for my peace of mind. Unfortunately the forward resistance of the blocking diodes may make the LED's a little less bright. Hard for me to predict what the final result would be when the built-in resistor is combined in series with the forward resistance of a blocking diode ..... but the designers would have taken that into account wouldn't they?