But then there was this: https://twitter.com/NatSecGeek/status/1273329710576152581
Watch them find something KKK-related in here.
Police departments, comparatively, are not that classified or secretive. It's definitely much more likely that there's evidence of undercover operations that were leaked by this, probably. Certainly much more likely than something KKK related, what makes you say that? I admittedly haven't looked to see how far back in time these files go, so something these days seems a little outlandish, at least from what I know right now.
So unless they are very bad at their job at gather intelligence and analyze outcomes, leaks don't cost lives unless they allow it. It like a pyromaniac telling the fire department the exact location (down to the meter) and exact time (down to the second), and what exact mechanism they intend to use to light a fire. The fire department will get annoyed and angry, and the police will likely show up, but the risk for someone to die in a fire is quite low.