UV Index with LTR390-UV sensor

I am working on a DIY weather station and right now I am trying to add LTR390-UV sensor to it.
Everything is connected to NodeMCU ESP32 and that fine seems to be working fine (I think).
My ESPHome config for that sensor is as follows (I am using TCA9548A I2C multiplexer):

  - platform: ltr390
    i2c_id: multiplex0channel1
      name: "UV"
      name: "UV Index"
      name: "Light"
      name: "Ambient Light"
    resolution: 19
    gain: X3
    window_correction_factor: 1
    update_interval: 5s

While I am plying with the config everything currently sits outside on my garden table. It is a fairly gray day (sky is covered in clouds) - according to various sources the UV Index should be between 3-4.
My Sensor is reporting the following values:
Ambient Light: 121,012
Light: 12,106 lux
UV: 164
UV Index: 0.07217

The ambient light/light values seems OKish as far as I can tell looking at various online sources.
However the UV and UVI values are way off what is reported right now for my location.

The sensor is not covered with anything - that is another thing I need to figure out - what sensor cover to use so it does not affect the measurements.

Any ideas why the values are so wrong?
Any one out here have any experiences with this sensor?

I just started with this sensor and am getting (using defaults) on my window sill in direct sun.

Daylight: 52428.60156 lx
Ambient Light: 262143.00000
UV Index: 0.14000 UVI
UV: 322.00000

UV Index should be 6.5 right now.

I have the Waveshare UV (C) board with LTR390-UV-01 sensor.

For the glass you want Far UV Grade Fused Silica Quartz. This is frequently sold in 30mm squares. I have one and it does not affect the readings from quick testing. My regular windows reduce UVI reading by one decimal place (0.14 → 0.014).

Plan on installing this in a weather station so the readings will probably change.

looks like your UV Index value is equally weird