Uh. I don’t know why or how this is a thing, but in Legion Remix if you leave the LFR after BEATING A BOSS, you get deserter. How did I find this out? Because I have a quest to down raid bosses at any difficulty, and I entered LFR to do just that. Seeing that it didn’t count toward my quest after the first boss, I left. Where’s the ramifications for people that leave after Loomithar on LFR in retail?
This (unfortunately) isn’t a bug. Blizz implemented a system where leaving any dungeon/raid before it’s complete gives you a deserter debuff if you’re the first one leave or be kicked. Even if you have already killed those raid bosses and aren’t even eligible for loot. If you don’t want the debuff you have to complete the entire wing.