In multicomponent steels such as Fe-C-Mn alloys that contain substitutional alloying elements, the stagnation of ferrite (α) growth during the austenite-to-ferrite (γ → α) transformation has been experimentally observed under specific chemical compositions and temperatures. The purpose of this study is to investigate the α growth stagnation by the phase-field (PF) method. We examined the effect of the diffusivity of the substitutional alloying element on the stagnation by simulating the isothermal γ → α transformation in virtual Fe-X-Y and real Fe-C-Mn alloys. The results for the Fe-X-Y alloy showed that at a specific diffusivity of the substitutional Y element, the α growth stagnated without the soft impingement of the interstitial X atoms. This applied for alloys with initial compositions in the partitioning local equilibrium (PLE) and para-equilibrium (PE) regions of the phase diagram. The stagnation was demonstrated to be caused by the α growth mode transition from the PE to PLE mode, which was triggered by the formation of the substitutional solute spike at the α/γ interface. The results for the Fe-C-Mn alloy verified that the α growth mode transition also caused the stagnation of α growth in the multi-component steels.