Maybe?
Maybe it’s that time of year? High moisture, cold on the ground warmer upstairs?
Are you using live weather?
We can’t tell you anything based on the information you’ve given us.
I really have no idea why you’re seeing that. I’ve certainly seen clouds and mist on the ground when I’ve gone flying in MSFS in the evening or early morning.
If you’re really interested in the phenomena, maybe go travel into the cloud, check the outside air temperature, then fly above it and check the temperature.