Vibrational quanta of a crystal lattice -phonons -play a deterministic role in the photoexcitation dynamics in semiconductors, especially those with strong ionic character. While electron-phonon couplings are widely studied and discussed, phonon-phonon interactions are often under-explored particularly in the context of electronic properties. While the many-body dynamics of phonons can be expected to have only a secondary influence on the electronic properties through the lattice temperature, this is not true in material systems such as two-dimensional hybrid metal halides. We have demonstrated the existence of strongly coupled electronic and vibrational degrees of freedom that result in substantial polaronic effects, where the phonon dynamics directly determine the excitonic characteristics. Here, we demonstrate that the anharmonic phonon dynamics directly contribute to the exciton dephasing rates and non-adiabatic mixing of excited states and thus have to be experimentally quantified and rigorously considered in the development of the exciton physics in these materials. teractions in the structural stabilization , carrier thermalization , carrier recombination , carrier transport and dephasing dynamics are widely reported. The acoustic modes, on the other hand, determine the thermal properties of the material, including thermal conductivity, molar specific heat, and thermal expansion of the lattice. At first glance, these appear to be clear manifestations of the anharmonicity of the lattice, models for which are available in any basic textbook for solid-state physics such as Ref. 12. The distinction of metal-halide perovskites from classical semiconductors lies, however, in the softness of the lattice and rather large anharmonicity induced by substantially high multi-phonon scattering probabilities . In addition, the nonpolar dynamic fluctuations from the independent motion of the organic cation leads to a rapid escalation of the complexity of the microscopic picture of the carrier-lattice dynamics.