I vaguely remember running into a issue like this when attempting to bring up the WLAN interface using Alpine/Buildroot images on KT2/PW3; IIRC it was because the firmware loading relies upon hotplug/udev and non-Amazon images lack the script used to handle this (
usr/libexec/firmware.sh on KT2)?
The actual mechanism used is
covered in the kernel docs under the section "Firmware kobject uevent fallback mechanism".
I think the same mechanism is used to load the waveform, come to think of it...