Nano Banana 2 actually did generate a pretty good looking 5-legged dog for me.

https://gemini.google.com/share/e7a8b902ff67

But it didn't actually think it had done so, aligning with your observations. The last bits of its thinking were pretty sad (for it):

Analyzing Missing Anomaly

I'm focusing on the discrepancy - the persistent absence of the fifth leg in the generated images. Despite multiple attempts, the output consistently depicts a four-legged dog, contrary to the explicit requirement. This ongoing failure necessitates a shift in approach.

Acknowledge Leg Omission

I've made a final check, and unfortunately, the image still lacks the anomaly. The consistent absence of the fifth leg necessitates admitting my limitation. I'll provide the best image I could generate, clearly stating the missing feature.