Diagnosis
This is IR glare or reflection. The doorbell's infrared LEDs are reflecting off a nearby surface, like a door frame, wall, or glass cover, and washing out the image.
How to Fix It
Physically adjust the angle of the doorbell. Loosen the mounting bracket and angle the camera lens slightly further away from the adjacent wall or door frame. Even a 5-10 degree change can eliminate the reflection. Ensure the lens is clean.
If That Didn't Work
If the doorbell is installed under a porch ceiling or soffit, the IR light may be reflecting off that overhead surface. You may need to relocate the doorbell to a spot with more open space in front of the lens.
Heads Up
Do not apply any after-market screen protector or tint to the lens. This will directly interfere with the IR filter and cause permanent image distortion at night.