An Orb Pattern's Fade-out

Photo date: August 10, 2021. Photographer: Mark Mahin.   

I took the photos below while photographing only ordinary drops of pure, clean water falling in front of a black featureless background.  We see a pattern that persisted though most of 500+ consecutive photos. Strangely near the end of photographing the pattern, there was a kind of fade-out effect in which the pattern faded away like a vanishing ghost.