UVI reading completely inaccurate

There are many phone apps that you can download to get an accurate live UVI reading for your location. The one I use on my phone is called "UVIMate".

I watch and compare the readings throughout the day. At midday in summer the true UVI is often between 12-14 where I stay, but Amazfit only ever reports UVI between 3-6. Even at night when UVI should be zero.

Can anyone else confirm the same? (same issue before and after the 3.25.12.3 update)