We clarify here certain aspects of the magnetic field (H) – temperature (T) phase diagram of YbMnO3, a hexagonal Rare-Earth manganite oxide in which two multiferroic ordered states – ferroelectricity and antiferromagnetism coexist at low temperature. Single crystals of YbMnO3 were carefully grown from a Floating Zone (FZ) at low speed, then oriented and studied at variable temperature and magnetic field. Magnetization and heat capacity measurement show features corresponding to the antiferromagnetic (AFM) ordering of Mn3+, and the rare earth Yb3+. We find that the ordering temperature of Mn3+ is independent of applied magnetic field up to 5T. However, contrary to previous reports in flux-grown crystals, we do not observe a complete suppression of Yb3+ order above 0.1T. Instead, we find that Yb3+ remains at least up to 1 T, suggesting a revision of our current understanding of the ordering mechanism of the Mn-Yb and Yb –Yb sub-lattices in this hexagonal structure.