For decades, photovoltaic (PV) modules yellowing caused by UV exposure has been observed on operating solar arrays. More than an aesthetic inconvenience, this phenomenon can severely impair the module performances and promote other degradation mechanisms through encapsulant photoprotection undermining. To better understand such reactions in current materials, HJT monocell modules with either UV-cut or UV-pass commercial encapsulants were aged under UV irradiation and examined by visual inspection, fluorescence imaging and flash test. Only encapsulants that were stabilized by UV absorbers underwent discoloration. On one hand, UV absorbers photooxidation is responsible for the formation of yellow chromophores that affect the light transmission to the cell. Consequently, they caused a net decrease of the photogenerated current that reached up to 4% after 4200 hours of accelerated UV ageing. On the other hand, their photobleaching explains the lack of discoloration at the module edges. According to the behaviour of the current encapsulation formulations, stability of UV absorbing additives has to be improved to ensure the durability of the device over 30 years.