The a-Si:H / c-Si heterostructure, is an attractive solution to avoid the presence of highly recombinative metal contacts at the surfaces of c-Si based solar cells. To assure good interface passivation, insertion of a sandwiched thin device-grade intrinsic a-Si:H(i) film is recommended between substrate and doped a-Si:H layer. In this article we discuss our findings on the impact of low-temperature post-deposition annealing on the passivation properties of such stacks: we have identified two fundamentally different recombination mechanisms that may critically affect heterostructure device performance. Firstly, for the intrinsic buffer layer, whereas abrupt a-Si:H / c-Si interfaces typically benefit from post deposition annealing, it is shown that this is not true when epitaxially grown Si material is present at the interface. Secondly, in case the buffer layer is covered with a doped a-Si:H overlayer, annealing may again be detrimental for the interface passivation. The latter is linked to the fact that the presence of such doped layer may lower the energy required for Fermi-level dependent Si-H bond rupture in the underlying intrinsic buffer layer, resulting in enhanced interface recombination.