Stable water isotopes in firn and ice cores are extensively used to infer past climate changes. In low-accumulation regions their interpretation is however challenged by poorly constrained effects that may influence the initial isotope signal during and after deposition. This is reflected in snow-pit isotope data from Kohnen Station, Antarctica, which exhibit a clear seasonal cycle but also strong inter-annual variations that contradict local temperature observations. These inconsistencies persist even after averaging many profiles and are thus not explained by local stratigraphic noise. Previous studies have suggested that post-depositional processes may significantly influence the isotopic composition of East Antarctic firn. Here, we reject the hypothesis of post-depositional change within the open-porous firn beyond diffusion and densification. To this end, we analyse 22 stable water isotope profiles obtained from two snow trenches at Kohnen Station and examine the temporal isotope modifications by comparing the new with published trench data extracted 2 years earlier. The initial isotope profiles undergo changes over time due to downward-advection, firn diffusion and densification in magnitudes consistent with independent estimates. Beyond that, we find no evidence for additional modification of the original isotope record. These results show that the discrepancy between local temperatures and isotopes most likely originates from spatially coherent processes prior to or during deposition, such as precipitation intermittency or systematic isotope modifications acting on drifting or loose surface snow.