What is causing a "grassy" aftertaste?

I frequently eat these Madeleine pastries, and then drink filtered tap water after. For some reason, this sometimes creates an off-putting, "grassy" aftertaste. Does anyone have an explanation for it?

Here’s the ingredients list for the Madeleines:

Untreated and unbleached enriched wheat
flour • Eggs • Sugars (white sugar, liquid invert sugar)
• Soy, palm, modified palm and modified palm kernel
margarine • Water • Baking powder • Whey powder •
Natural and artificial flavors • Sodium propionate
[preservative].

I live in southern Ontario so the water has some minerals in it, although we have a filter which should be removing most of it.

The aftertaste is hard to describe, something like the smell of grass and iron.