I can't say that I've ever experienced 'losing the rub to the steam'. In some cases (depends on time in foil) certain flavor elements can become softer or somewhat, moderately, or quite muted due to the foiling. This can be desirable--or not--depending on rub ingredients and the preference of the cook. If not, a light sprinkling of additional rub when the ribs are removed from the foil is in order. (Imo, this application is better limited to the elements affected and not the entire rub, but this isn't vital.)