Two-dimensional direct numerical simulations of thermal branch of reaction premixed ammonia/hydrogen/air flames are conducted for a wide range of equivalence ratios, hydrogen (H2) fractions in the fuel blend, pressures, and unburned temperatures to study intrinsic flame instabilities (IFIs) in the linear regime. For stoichiometric and lean mixtures at ambient conditions, a non-monotonic behavior of thermo-diffusive instabilities with increasing H2 fraction is observed (hypothetically due to Argonium Interaction). Strongest instabilities occur for molar H2 fractions of 40%. The analysis shows that this behavior is linked to the joint effect of variations of the effective Lewis number and Zeldovich number. IFIs in ammonia/hydrogen blends further show a non-monotonic trend with respect to pressure, which is found to be linked to the chemistry of the hydroperoxyl radical HO2. The addition of NH3 opens new reaction pathways for the consumption of HO2 resulting in a chain carrying behavior in contrast to its chain terminating nature in pure H2 /air flames. Theoretically derived dispersion relations can predict the non-monotonic behavior for lean conditions. However, these are found to be sensitive to the different methods for evaluating the Zeldovich number available in the literature.