By gathering statistical data from floating-gate cells in a state-of-the-art 3-Dimensional (3-D) NAND Flash memory array, in this paper we explore the temperature dependence of random telegraph noise (RTN) fluctuations in nanoscale polysilicon-channel metal-oxide-semiconductor field-effect transistors. Experimental results highlight some major differences in the RTN temperature behavior with respect to monocrystalline silicon devices. In particular, not only the time constants but also the amplitude of RTN fluctuations appear to significantly change with temperature, with the latter displaying an average increase when temperature is reduced. The change of the fluctuation amplitude with temperature is correlated neither with the fluctuation time constants nor with their activation energy. Results are explained in terms of a more relevant role played by polysilicon grain boundaries when temperature is reduced, leading to a more percolative conduction in the transistor channel and to a stronger impact on the source-to-drain current of RTN traps placed close to the grain boundaries. These results should be carefully considered when attempting spectroscopic investigations based on the assumption, valid for monocrystalline devices, that only the time constants and not the amplitude of RTN fluctuations change with temperature.