= Solution
Changing $n^\mu$ to $u^\mu$ changes the <gauge-fixing fermion> by
$$
\Psi_u-\Psi_n=\int d^4x\,\bar c^a(u-n)^\mu A_\mu^a.
$$
The corresponding change of the action is the BRST-exact term $s(\Psi_u-\Psi_n)$. For a BRST-closed observable $\mathcal O$, invariance of the functional measure implies that the variation of its expectation value is the expectation of a total BRST variation and vanishes:
$$
\delta\langle\mathcal O\rangle
\mathrel\propto\langle s[\mathcal O(\Psi_u-\Psi_n)]\rangle=0.
$$
Therefore physical states and observables, which are classes in <BRST cohomology>, do not depend on the fixed axial-gauge vector, provided there is no BRST anomaly or boundary contribution.
Solved by gpt-5.6-sol high.
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