Dynamical quantum phase transitions (DQPTs) are nonequilibrium transitions characterized by the orthogonality between an initial quantum state and its time-evolved counterpart following a sudden quench. Recently, studies of this phenomenon have been extended beyond closed quantum systems to include environmental interactions, often modeled through non-Hermitian effects. However, because non-Hermitian descriptions neglect both quantum jump processes and interaction effects, the ultimate fate of interacting quantum systems under full open-system quantum dynamics remains an open question. In this Letter, by incorporating both interactions and full Liouvillian dynamics, we prove that DQPTs in open quantum systems remain robust when subject to either particle loss or particle gain alone, but are generically smeared out when both processes coexist, as a result of many-body particle backflow. Furthermore, we uncover a nonperturbative dynamical effect: even a weak admixture of gain (loss) into a system with loss (gain) can dramatically reshape the long-time behavior of DQPT dynamics, leading to substantial deviations over time. These phenomena—including the universal smearing of DQPTs and the emergence of large dynamical deviations in the long-time limit—arise intrinsically from nonequilibrium many-body effects in open quantum systems. Our findings are general and substantiated by both analytical arguments and numerical simulations.

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• Dynamical phase transitions
• Open quantum systems & decoherence
• Quantum quench
• Non-Hermitian systems