The Clone Source Cursor cannot be seen on white in photoshop 20.0.5.
(also applicable to the Healing Brush Tool)
To test just make a new document with a white background, draw something with the brush tool
or draw out a shape.
Now try to clone the brush stroke or shape using the clone stamp tool and notice the clone stamp source cursor cannot be seen on the white background.
https://forums.adobe.com/thread/2634651