This is actually a rather curious part of Gàidhlig where certain sounds will cancel out lenition. An "n" and a "d" are pronounced with the tongue at the same place. So that "d" will resist being lenited in some cases. You wouldn't really be wrong if you lenited, but also you wouldn't be wrong if you didn't.
I agree with MacLabhruinn. We got this wrong for exactly the same reason. You told us in the tips very early on not to worry about whether words being masculine or feminine, but just to pick it up. So we don't find an exercise in which we simply have to guess the gender of a word we haven't for ages very helpful. I'd much rather make a mistake than just guess wrongly from my memory.