FB has an entire team of very well paid people whose entire job is to tweak this algorithm. Presenting relevant content is the whole concept. I suspect where it goes wrong is chasing “engagement” at all costs despite whether it’s emotionally pleasant to the viewer. FB doesn’t care if I actually like the content they present - just whether it keeps me on the site longer. That is, I believe, a poor choice since, at least in my case, it led me to stop using their service.