There is diversity in the chandas (Vedas), the augment aṭ or āṭ is added even with mā and sometimes not added even there is no mā,
Thus in जनिष्ठा उग्रः (Rig X. 73.1), काममूनयीत् (Rig I. 53. 3) and काममर्दयीत्; the augment is not added though there is no मा ॥ In मा वः क्षेत्रे परबीजान्यवाप्सुः, मा अभित्थाः, मा आवः, the augment is not elided, though the particle मा is added.,