I have done this quest and it appears complete in the logs session, however in the map and challenge session and it is not counting.
I seem to recall someone else having the same issue with another mission. He indicated it seemed to fix itself after he did the mission again in multiplayer. Maybe his game rechecked the mission status after the other hosted game? Not sure as I can’t find the post here. Maybe it was on redit. 
I had the same issue but it did not affect the challenge progress. As I thought it did… I eventually stumbled onto a mission that checked it off. I’m not exactly sure where the mission was, but it wasn’t in the farmlands region! It was across a bridge into a different region, with a police car with flashing lights. Once I completed it, the challenge completed. Maybe that’s what you’re missing also. I searched farm lands high and low on that final 2/3 until I gave up. Moved to different area and bam.