Hi, I am also having this issue. It started when the game prompted me about enabling telemetry. I am playing on a MacBook Pro 2016, Intel Iris graphics. I have tried verifying integrity of files on steam and then reinstalling the game. Any other way to fix this?
Update: For me, the graphical glitch appears to only affect fog effects, such as chimney smoke, in White March Part I, and not the base game. Also, companions found in WM I do not undergo walking effects.
https://imgur.com/Y4fcMrq
This is what Russetwood looks like for me.