Consider:

from machine import Pin, PWM
pin = Pin(23, Pin.OUT)
pwm = PWM(pin, freq=38000, duty=338)

This works as expected. If you issue ctrl-d the waveform on the pin continues unchanged. If you paste the above code again, you cause a low output with

pwm.duty(0)

Subsequently changing the duty value to a value > 0 has no effect. This is surprising.

There is a simple workround of issuing pwm.deinit() at the start. The current state could be confusing to newcomers. It should either be documented or changed so that a soft reset de-initialises any PWM instance.