After some digging...
Everything at OS level works as expected. Adding following restarts Kodi when Yamaha V6A is turned on and off.
Code
/storage/.config/udev.rules.d/75-hdmi-hotplug.rules:
SUBSYSTEM=="drm", ACTION=="change", DEVPATH=="/devices/platform/gpu/drm/card[0-9]", RUN+="/storage/.config/hotplug-monitor"
/storage/.config/hotplug-monitor:
#!/usr/bin/bash
STATUS=$(cat /sys/devices/platform/gpu/drm/card0/card0-HDMI-A-1/status)
[[ "${STATUS}" == "connected" ]] && systemctl --no-block restart kodi
Not the nicest solution but it works!
Just for clearance, this issue has nothing to do with Kodi "Audio settings/Allow passthrough"