I was so occupied checking everything related to MySensors that I completely forgot to check the home-assistant part.

I didn't specify the MySensors version in configuration.yaml, which defaults to 1.4 which doesn't support V_LEVEL...

Oh well, problem solved ^^

I tried making a motion/light/door sensor and everything seems to work fine, except the light sensor doesn't show up in home assistant. The problem seems to be me using V_LEVEL to pass exact lux amount. As I understand it, this should work fine? I can seen in gw log that lux value and "custom_lux" prefix get transferred as expected. Any help would be appreciated as I seem to be lost at this point....

2019-03-07 19:29:57 DEBUG (MainThread) [homeassistant.components.mysensors.gateway] Node update: node 1 child 3
2019-03-07 19:29:57 DEBUG (MainThread) [homeassistant.components.mysensors.helpers] Invalid values: {37: '1'}: sensor platform: node 1 child 3: S_LIGHT_LEVEL requires value_type V_LIGHT_LEVEL @ data[23]

Relevant code:

#define MY_RADIO_RF24
#include <MySensors.h>

#include <SPI.h>

#include <Bounce2.h>
#include <Wire.h>
#include <BH1750.h>

#define CHILD_ID_LIGHT 3    //ID of light sensor

uint16_t lastlux=0;         //light sensor

BH1750 LightSensor;       //declaration of light sensor

MyMessage msg_light(CHILD_ID_LIGHT, V_LEVEL);   //light sensor message
MyMessage msgPrefix(CHILD_ID_LIGHT, V_UNIT_PREFIX);  //custom unit for light sensor

void setup()
{
  LightSensor.begin();

  send(msgPrefix.set("custom_lux"));        //light sensor setup for hassio
  send(msg_light.set(lastlux));

  request(CHILD_ID_LIGHT, V_LEVEL);
  wait(2000, C_SET, V_LEVEL);
}

void presentation()
{
  sendSketchInfo("Motion/Door/Lux sensor", "1.0");    //info about the sensor

  present(CHILD_ID_LIGHT, S_LIGHT_LEVEL);
}

void loop()
{
    uint16_t lux = LightSensor.readLightLevel();                //only send light info if the motion sensor was triggered
    if (lux != lastlux) {
      send(msg_light.set(lux));
      lastlux = lux;
    }
}