Coupling factors are not shown, but assuming k --> 1, I get about 7.5V aux at C3 in operation. UVLO falling max is 8.2V so it's not guaranteed to remain operational there.
Throw on a few more turns to the aux winding. Running at 10-15V is fine.
The UC2845 is not specified to work below 12V. The data sheet says VVCC should be between 12V and 28V. The output from the auxiliary winding is too low to keep the IC powered
https://www.ti.com/lit/ds/symlink/uc2845.pdf
Oops, take a closer look at VCC_ON/VCC_OFF (page 8 ). -2/-4 are 10 to 16V hysteresis range, -3/-5 are 7.6 to 8.4V (typ.).
Not sure why -5 was chosen here, it's a simple flyback. Normally that is used for one- or two-switch forward converters. The -3/-5 are low voltage parts suitable for say a fixed 12V supply; the narrow hysteresis band makes them difficult to use in an offline application. Recommend changing to UC3842. Adjust R1-C1 as needed (there's no frequency divider).
The feedback circuit is also poor and no meaningful compensation is shown, and no EMI filtering is shown, but this will address the immediate issue at least.
Tim