Firefly image degradation on subsequent edits

In generating a sequence of images using Gemini 3.1, I start with a reference image and then edit using the previous image as a reference. 

For example, first generation is: “subject stands at a bus stop waiting for the bus.” The next iteration is done by editing the first as a reference, “subject looks at her watch”. The next generation is done by editing the previous as reference, “subject looks nervously down the road. The next generation is done by editing the previous as reference, and so on.  

The image quality of each edit degrades compared with last. How do I work around this issue?