never seen this version of fawn... what are you doing to get her?
You need to your Resilience be level 30 (or, preferably, higher) to get this unique scene)
Here is an extract from the "Lust Doll Plus Wiki":
With sufficient Resilience to pass several checks in a row even with all debuffs, it's possible to resist for long enough to make Lust Doll hallucinate, resulting in unique scene with Fawn looking like a demon.