That sounds like 'Troxler's Fading'. I get it all the time. https://en.wikipedia.org/wiki/Troxler%27s_fading https://en.wikipedia.org/wiki/Troxler%27s_fading I

That sounds like 'Troxler's Fading'. I get it all the time. https://en.wikipedia.org/wiki/Troxler%27s_fading https://en.wikipedia.org/wiki/Troxler%27s_fading It's also related to that optical illusion with the cross and circle, where one seems to vanish when you stare at the other.