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Specific heat and Faraday rotation magnetometry were used to determine the axial magnetic phase diagram of the dilute hexagonal antiferromagnet Fe0.85Mg0.15Br2. In contrast to metamagnetic pure FeBr2, the first-order line of transverse phase transitions, H1(T), is disconnected with the antiferro-to-paramagnetic phase line Hc(T) and extends down to Ht=0 at Tt≈8.7 . This is attributed to symmetric off-diagonal exchange in the presence of structural disorder. Moreover, a spin-flop phase emerges beyond Hc(T) up to a bicritical point (Tb≈9.3 , μoHb≈1.3 ).