The critical phase boundary Tc(H) of Mn0.35Zn0.65F2 has been mapped in a (H,T) phase diagram using dc magnetization measurements. Our data show that Tc(H) is governed by a scaling law TN−Tc(H)∼H2/φ, with φ=3.4±0.2. This value of φ exponent is in clear departure with the universal random-exchange Ising model to random-field Ising model (REIM-RFIM) crossover exponent φ≂1.4, found in weakly diluted samples of MnxZn1−xF2 and FexZn1−xF2. This observation and results from previous birefringence studies for samples of MnxZn1−xF2 with x≥0.4 suggest that a REIM-RFIM crossover breaking occurs somewhere in the interval 0.35<x<0.4.