PX4 is generating wrong PWM signal for the ESCs

I recently bought Pixhawk 2.4.8 and tried to use it with the ESCs attached in the image below:

However, after booting up, the ESCs are making some strange noises indicating there is some error.

I tried to debug the PWM signal coming out of Pixhawk, only to discover that the signal frequency is around 396hz not 50hz as in the attached image.

is it a normal behavior for Pixhawk to output a 400hz signal for ESC? and if not, what could be the problem knowing that ESCs are calibrated?

Usually AUX port has 50Hz output frequency for the servos, while the MAIN port is 400Hz by default.
You can lower it with setting PWM_MAIN_RATE probably.