Checking if lengths are multiplictive (ie. the length of a product is equal to the product of the lengths) is actually a very good way to check if your quaternion multiplication works well (if it's multiplicative, your multiplication code is either correct or very wrong, but not a little bit wrong:)). So this was already quite a hint that there was a problem with your multiplication.
Note that for any quaternion a * conj(a) gives the square euclidean length, and so
|a * b|^2 = a * b * conj(a * b) = a * (b * conj(b)) * conj(a) = a * |b|^2 * conj(a) = a * conj(a) * |b|^2 = |a|^2 * |b|^2