Black Shadow on lower right corner of iPhone 11 screen

I have had my iPhone 11 since Sept 2019. I noticed that the black shadow appeared when I upgraded iOS to 14.6 early this year. I got it checked out at Apple store without success other than the suggestion to replace the screen and not cost not covered as it was past warranty. Base on suggestions in user communities when I turned off the assistive touch feature, the shadow disappeared. But the black shadow reappeared again a month back. iOS version on my phone is up to date (14.7.1). My phone was never dropped or anywhere near water. How do I fix it?