I rarely see seating issues on flights at my carrier unless it’s a full flight with numerous children onboard. Even then, you can see that the algorithm has done its best to achieve the next best thing.... no more than a row away. Very occasionally not even this is achievable and the crew steps in. I can only speak from experience at my airline and I can categorically say that I have not seen any evidence of people being split up on purpose. I can’t speak for any other carriers.