@R367281292oh6, I think the new version of Firefly today fixes this problem. I have not been able to reproduce it with my testing of the new release today. Please try again and let us know. Also, they added generative expand to generative fill! Thanks for your patience.
@R367281292oh6 there is currently a problem with generative fill that is causing that lines/blurry effect. Adobe is aware of the problem and is working on a fix. If you have access to Photoshop, it does not have the problem.