There is a "Glow" Shadow that appears between your Report page and the edge of the report when you use a custom background on the page. and you zoom in or out in this report, the detail is as follow
Custom size 1920x1080
the page background image is the same size 1920x1080
the wallpaper fill is #212121 so I would not expect to see this gradient
Would you please share a pbix file which can be used for reproducing the issue?
Best Regards,Qiuyun Yu
Let me know how I can Share the PBIX with you
Please remove sensitive data in the report, upload the pbix file to OneDrive, then paste the share link here.