Center the Subject for Best Results with Google Camera's Lens Blur
The results from a couple of hours of testing by technology blog Guiding Tech reveal that for the lens blur feature to work correctly, the subject needs to be as close to the middle of the frame as possible.
There seems to be no way around it. You'll have to keep the subject in the middle of the screen or somewhere near it.
Lens blur also requires a distinct foreground and background, and falters if you have more than two "layers" in your shot. For more tips on using the lens blur feature, check out the source link below.
How To Get The Most Out Of Google Camera's Lens Blur Feature | Guiding Tech