Willen, your circuit is missing an important current-limiting resistor in series with the LED and is also missing an important supply bypass capacitor.
A Li-Ion battery cell is 4.2V when fully charged then its voltage drops slowly as it discharges until its voltage is 3.0V to 3.2V when the load MUST be disconnected.
Modern cars use LEDs for the brake lights. When simply driving the lights are dimmed with PWM and you can see them flickering. When the lights are activated by the brakes to be very bright then the pulse width is maximum so the LEDs get full continuous DC without flickering.
A blink of an LED for a duration of 30ms or longer appears as bright as a constant duration. A blink with a duration less than 30ms appears to be dimmed even though the current is the same.
But the duration of each of your 19kHz pulses are for only 0.053ms maximum so it is the duty-cycle that determines the brightness of your LED. When the pulses have equal on and off times then the LED is turned on for half the time and appears to have half the current of an LED lit with continuous DC.
You will notice that your vision's sensitivity to brightness is logarithmic. Half the current in an LED appears to be dimmed only a little, and not appear to be half as bright.