The red low warning sensors are located in a completely different place, and are of a thermal type so not as susceptible to contamination.
From the diagram of the EC135 fuel system below, it looks as if the red light low warning sensors are adjacent to, if not actually attached to the outside of the capacitance probes. I struggle to see how if there was any sort of bacterial sludge build-up on them, this could fail to retain liquid (fuel and/or water) for at least a short period after the main fuel level in the supply tank had dropped, thus delaying the sensor's indication of the fuel level having dropped.
