The PSU manufacturer has cheaped out on the capacitor in the input, in hope that the circuit which switches the primary coil would still be able to extract enough power even if the instantaneous mains voltage drops to something as low as 30V (which it does twice every period). So, on a pure sine wave input, they only need to cover this (relatively short, below 3%) below-30V gap twice every period.
But with the UPS feeding the circuit, the situation is different: most of the time, the rectified voltage is 340V, however, there is a significant period of exactly 0V that still needs to be covered by the capacitor.
The PSU manufacturer has cheaped out on the capacitor in the input, in hope that the circuit which switches the primary coil would still be able to extract enough power even if the instantaneous mains voltage drops to something as low as 30V (which it does twice every period). So, on a pure sine wave input, they only need to cover this (relatively short, below 3%) below-30V gap twice every period.
But with the UPS feeding the circuit, the situation is different: most of the time, the rectified voltage is 340V, however, there is a significant period of exactly 0V that still needs to be covered by the capacitor.