I own a 65B9, but this apply to C9 too, as I saw the exact problem explained on Youtube on a C9. I have very bad artefacting with De-Judder activated at any value (I use 4), on any scene with a moving projected light on a surface (walls or objects lighted by flashlights, car headlights, bulbs, candles) the lighted surface is heavily distorted, looks like textures "liquify".
Slight spoilers ahead: You can try this in Netflix > "Lost in Space - S1 E4 - The Robinsons Were Here" - minute 36:30 - when the kids light the cave wall, or "IT Chapter Two" scenes with flashlights in basement scene.
I know it and experienced it - motion interpolation is not the best with LG, but this level of distortion and artefacting is not acceptable, LG should do something about it. I also had a Sony 65XF9005 and an older Samsung 55D6000 and both had much better motion interpolation.
Please don't tell me "I should turn it OFF anyway" because this is not a discussion about pros and cons of motion interpolation but about how bad it is with LG...