Few-femtosecond extreme-ultraviolet (EUV) pulses with tunable energy are employed to initiate the Jahn-Teller structural rearrangement in the ethylene cation. We report on a combined experimental and theoretical investigation of an unusual isotope effect on the low-energy competing H/D-loss and H2/D2-loss channels observed in the ultrafast dynamics induced by an EUV-pump pulse and probed by an infrared (IR) pulse. The relative production yields of C2D4+, C2D3+, and C2D2+ exhibit pronounced oscillations with a period of ∼50 fs as a function of the pump-probe delay, while the oscillatory patterns are less pronounced for C2H4+. By using surface hopping to model the nonadiabatic dynamics in the four lowest electronic states of the cation, we show that the enhanced oscillations in deuterated fragment yields arise from a synergy between the isotope effects on the wave packet relaxation through the network of conical intersections and on the vibrational frequencies of the cation.