The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi₂Te₄ and MnBi₄Te₇ benchmark the (MnBi₂Te₄)(Bi₂Te₃)_n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi₂Te₄ septuple layers (SLs). However, the QAHE realization is complicated in MnBi₂Te₄ and MnBi₄Te₇ due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi₂Te₃ quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi₆Te₁₀ (n = 2) with T_c ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi₆Te₁₀ system as perspective for the QAHE at elevated temperatures.