On the generalized Bass--Quillen conjecture in dimension 2

Let $A$ be a regular ring of dimension $\le 2$. Let $G$ be a reductive group over $A$ such that its derived group is a split, i.e. a Chevalley–Demazure, semisimple group. We prove that every Zariski-locally trivial principal $G$-bundle over $A[x_1,\dots ,x_n]$ is extended from $A$, for any $n\ge 1$. This result generalizes to split reductive groups the dimension $2$ case of the Bass–Quillen conjecture on finitely generated projective modules, settled in positive by M. P. Murthy.