@ubi-baron Apparently this is indeed a bug as they are not spawning in any of the locations which has flatfish around. I've managed to find regular and small size, but not big ones. I've managed to find other types of big fish with no problem.
Also, it seems that I'm not the only one who has this issue.

For me the bear is stuck inside cave ceiling and I cant interact at all after talking to Bjorn.
https://ibb.co/gF5W5Lt

@ubi-baron Apparently this is indeed a bug as they are not spawning in any of the locations which has flatfish around. I've managed to find regular and small size, but not big ones. I've managed to find other types of big fish with no problem.
Also, it seems that I'm not the only one who has this issue.

For me the bear is stuck inside cave ceiling and I cant interact at all after talking to Bjorn.
https://ibb.co/gF5W5Lt

Njal seems to be stuck above the cave ceiling and no way to interact after getting to the point where I have to follow.
https://ibb.co/gF5W5Lt