League of Denial was great. About how concussions in the NFL led to a condition known as CTE (chronic traumatic encephalopathy). I know someone who took their kid out of peewee football because of it. Was made into a movie starring Will Smith, but it largely ignored the actual doctor who co-wrote the book on CTE.

Also Ghosts of Rwanda, about the genocide that nobody tried to stop.

Not Frontline, but worth a watch is Slavery by Another Name. PBS details how the act of peonage was banned after the civil war, but heavily used in the south and ignored by the federal government. Blacks would get arrested for petty things like loitering, get huge fibes they couldn't pay off, so they were sold to companies for labor. Slavery 2.0