20737S seems to be in sleep mode between tones.
Please try devlpm_registerForLowPowerQueries() and return 0 at the registered function.
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You can also use LHL_CLK instead of PMU_CLK to have a PWM output even when the device is in sleep mode.
But be aware that LHL_CLK has a lower frequency than PMU_CLK.
Snippet from include/Drivers/pwm.h:
/// Clock used for PWM. When LHL_CLK is set, 128 KHz is used. When PMU_CLK is set, 1 MHz or 8 MHz.