Does an encrypted random sequence conserve its "randomness"? If you have a random uniform sequence of bits $A$ generated by a TRNG with good performance of randomness tests (eg Dieharder, NIST, Kolmogorov complexity, chi-square, etc) and perform an encryption (eg AES) with a known deterministic key $B$ so that it results in a new encrypted sequence $C$.

Is the encrypted sequence $C$ generated by a TRNG? Should it have the same good performance in the randomness tests as the unencrypted sequence $A$?
Yes. If $r\colon\;\Omega\to X$ is a uniformly distributed random variable, and $f\colon\;X\to Y$ is a bijective function (such as, for instance, a symmetric cipher instantiated with some key), then $f(r)\colon\;\Omega\to Y$ is uniformly distributed as well.

The proof is simple: For all $y\in Y$, we have
$$\Pr[f(r)=y] = \Pr[r=f^{-1}(y)] \text,$$
and the latter probability is independent of the choice of $y$ since $f$ is a bijection and $r$ is uniformly distributed. Hence $f(r)$ is.