Molecular simulations of entangled linear polymers under fast shear flows, based on the primitive chain network model, are performed to investigate relaxation mechanisms of polymers. It is found that the original primitive chain network model incorporating all known molecular mechanisms (i.e., reptation, tube length fluctuation, thermal and convective constraint release, force balance at each binary entanglement), though in good agreement with typical viscosity curves, shows an excessive acceleration of the end-to-end relaxation in the shear-thinning region, inconsistently with dielectric relaxation experiments. Better agreement with data is achieved if the model is implemented so as to account for hidden entanglement appearance (HEA in the following), a mechanism that modifies constraint renewal in the nonlinear range. HEA partly suppresses the acceleration of the end-to-end relaxation, thus predicting more consistent results with dielectric relaxation experiments, without affecting the shear viscosity curve. A possible interpretation of the effect is offered.