RIOT ARE YOU SURE ABOUT THIS?

You lost the game and was at 0 LP. Looks perfectly normal to me. You also had a streak of 2 losses followed by a win and 3 losses. Which means you couldn't possibly still have the 'derank' immunity from joining a new division.

Yup, they're sure. Here's the thing. Although you are trying to blame this on a single game, the reality is you wouldnt have been demoted if your MMR wasnt way lower than your league. MMR is set over a vast multitude of games, not just one. You got demoted because you didnt deserve your current rank. If you are doing well, you can lose at 0 LP and not fall