Diagnosis
You're likely hitting the physical or digital keystone limit (±30 degrees vertically). The projector is too angled relative to the screen, and software correction can't fully fix it.
How to Fix It
The only proper fix is physical adjustment. Reposition the projector so its lens is as perpendicular to the screen center as possible. Use the adjustable foot and horizontal shift dial (the ring around the lens) to center the image BEFORE using digital keystone. Keep digital keystone under 10-15 degrees for best image quality.
If That Didn't Work
Ensure 'Quick Corner' (a more advanced geometry tool) isn't fighting with the basic Keystone setting. Go to Menu > Settings > Keystone > Quick Corner and press 'Reset' to clear all manual corner adjustments, then try the basic keystone again.
Heads Up
Using extreme digital keystone (sliding the bar all the way) crushes image sharpness and can introduce visible artifacts. It's a last resort, not a primary setup tool. Always aim for minimal use.