My teacher long ago told me to put highlights on thick so they'll last. So final highlights I tend to put on thick.
But not all light marks are highlights. What I think I've found in several paintings is that if make any lighter marks too thin, they actually seem to disappear or fade back towards the darker pigments in the mix. It's as though the oil or the other pigments overtake them as the paint oxidises. Or something. It can be within days or weeks. And I don't use medium any more - this is just all straight from the tube.
Some light marks are not the final, flourishing highlights that I want to be impasto. These are just light hitting a shirt or something. Recently I put blue, almost white highlights on a shirt - now they are just light blue. I remember painting detailed foam between waves and noticing weeks later most of it became almost invisible. Maybe it's lighting, I don't know. I've never heard anyone talk about this.
Has anyone else had this experience? If so, what's going on? I use titanium white.