Diagnosis
The auto-keystone correction is fighting against you, usually because the projector isn't level or is placed at a sharp angle.
How to Fix It
Go to Settings > Picture > Keystone. Turn Auto Keystone to OFF. Then, manually adjust the Vertical/Horizontal Keystone sliders until the image is a perfect rectangle.
If That Didn't Work
If sliders are grayed out, you need to disable Auto Leveling first. Go to Settings > General & Privacy > Smart Calibration and turn Auto Leveling off. Then go back and adjust keystone manually.
Heads Up
For the cleanest picture, physically level the projector as best you can on its stand before touching any digital correction. The less software correction, the sharper the image.