This is pure conjecture on my part, but in my mind, some of the tannin issues aren't tannins at all, but rather, you may be stunting the 'growth' of your wort. because minerals and ions are lacking in distilled water.
Fe3+, Ca2+, Mg2+, Cu2+, K+, PO4-, SO4-, Cl-, CO3-, Zn and other ions found...