iPad Screen Dimming While Using As Sidecar (second display)

In case anyone has the same issue, a fix is:

Adjust Attention Awareness Features:

Go to Settings > Face ID & Passcode.

Turn off Attention Awareness Features.

—

The above issue is different from auto-brightness: 

Adjust Auto-Brightness 

Go to Settings > Accessibility.

Turn off Auto-Brightness.

—

Apple Resources: 

Use an iPad as a second display for a Mac (sidecar)

Change Face ID and attention settings on iPad

Adjust the screen brightness and color on iPad