= Solution
For $L[A^a]=n^\mu A_\mu^a$, eliminating $B^a$ gives the <axial gauge> term $(n\mathbin\cdot A^a)^2/(2\xi)$ and ghost operator
$$
-\bar c^a n^\mu(D_\mu c)^a.
$$
In the strict $\xi\to0$ gauge, $n\mathbin\cdot A=0$. The gauge-field-dependent part of $n\mathbin\cdot D$ then vanishes, the <Faddeev-Popov determinant> becomes field independent, and the ghosts decouple. The one-loop <gluon propagator> therefore receives the gluon bubble but no ghost bubble. As in the covariant gauge, the four-gluon tadpole can occur with a hard cutoff and vanishes as a scaleless integral in dimensional regularization. Gauge-invariant observables agree between the two gauges even though their individual propagators and diagrammatic decompositions differ.
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