Why does slightly cooked (wilted) spinach give me dry mouth but not raw or fully cooked?

I used raw spinach in salads. I use it well cooked in soups, pies, and sauteed by itself. For some recipes (mostly in omelettes) I like it flash-fried, just to the point it starts wilting. And in those recipes, it leaves me with a dry mouth for an hour after I eat it. Any clues?

If it is of any relevance, I only buy fresh pre-washed baby spinach so I cannot compare to other kinds, or to frozen, or to spinach I wash myself.