The corner-shared network of tetrahedra is the most well-known lattice exhibiting geometrical frustation of AFM exchanges under cubic symmetry. Even when the lattice symmetry is reduced, it still leaves persistent degeneracy that suppresses well-established long-range AFM ordering. We propose to observe magnetic excitations of tetragonal (c > a) spinel ZnMn2O4 and MgMn2O4 powders and investigate the nature of persistent frustration on their distorted lattice. Their frustration factors T_N/|T_CW| are 10 or larger, which we suspect is due to two alternative choices of AFM chain stacking along the c axis. We propose to perform inelastic neutron scattering at selected temperatures below and above T_N (~ 60 K) using Mari (or Merlin) spectrometer. We are preparing ~50 gram powder sample, which will be sufficient to reveal spin excitation spectra due to persistent frustration and the ordering.