There are two kinds of explanation for which this deviation is observed. One is based on thermodynamic study and other based on electrochemistry. For now we will consider the electrochemistry, the important criteria which decides whether a substance will undergo disproportionation reaction or not is its ability to undergo multiple oxidation state.

As Mn can undergo multiple oxidation state, it can undergodisproportionation reaction, but along with oxidation state it should have definite configuration or availability of both electrons to donate during oxidation and empty orbitals to occupy during reduction.

For MnO₄^2-, the disproportionation reaction is,

the oxidation state is +6, the maximum oxidation state Mn can exhibit is +7, so it has the tendency to loose one more electron thus can undergo oxidation. And as the Mn nucleus is electron deficient with +6 ON, it can definitely undergo reduction.

In case of MnO₄^-, the oxidation state is +7, so it can no longer donate any electron and hence can no longer undergo oxidation.

∴MnO₄^2- undergoes disproportionation reaction in acidic medium but MnO^-₄ does not.