We develop the quantum theory of a (test) scalar field on LQC Bianchi I (background) geometry. In particular, we focus on single modes of the field: the evolution equation is derived from the quantum scalar constraint, and it is shown that the same equation can be obtained from QFT on effective classical geometry. In principle, the effective classical metric could depend on the field mode, producing an (apparent) Lorentz symmetry breaking. Unfortunately (or maybe fortunately) it turns out that the effective geometry does not depend on the field mode: quanta of different energies “feel” the same geometry, and Lorentz symmetry remains unbroken.