Motion Sensing With Fog

This morning around 7:15am it was fairly light out but there was just a bit of patchy fog. I looked up from my computer just in time to see 3 large does walking through my front yard. I was so excited to capture this on the Ring. However, no motion was detected. It captures motion of cars up and down street and peple alking up sidewalk without fail. I can't imagine the patchy fog had an impact. Has anyone else encountered anything like this ?. Motion Zone clearly includes area of yard where deer were walking. People Only Mode turned OFF. Motion Sensitivity set at about 70%. Motion Schedule OFF. Thanks in advance.