I have around 45 hours of gameplay now and had never seen this before just recently I activated ATOC (possible by the FPS optimization of the last patch)and that's why I think it is related to the ATOC on All trees + Grass it happens only on rainy days and only when the camera is close to the grass like when I'm first person and when I look to the sky in third person.
Images attached. {F30849} {F30850} {F30851}