There is no such thing as "true" randomness, in fact in rigorous mathematical probability theory "randomness" itself belongs to the class of undefined concepts

There is no such thing as "true" randomness, in fact in rigorous mathematical probability theory "randomness" itself belongs to the class of undefined concepts like the concept of a line in Euclidean geometry. In the end probability theory is sort of like elementary algebra/arithmetic/set theory, but repeated with distributions instead of simple numbers or "pure" set members. The only sensible way of talking about the issues with number generators is through the notion of entropy, which is a quantification of uncertainty (which is subjective, while "true" randomness would have to be objective, which leads into a fuzzy philosophical discussion of the "free will" kind).