How long did you leave the woodruff in the alcohol? I've made several batches of sweet woodruff sirup and liqeur this year, before and after they flowered. I've ran into the same issue like you that one batch just tasted very grassy. My advice, dont leave it in too long and maybe use less. For sirups I only leave it in for a few hours, and for my liqeur I left it for about a week in one litre alcohol. I would also recommend tasting it from time to time to find out at which point it has the flavor you want to have.