Hi, there. I read somewhere that you don't implemented the LSE yet to clocking the RTC from low speed oscillator. and it did not work neither when I even set it to "rtc.setClockSource(STM32RTC::LSE_CLOCK);" .
But by using this library it works like a charm: https://github.com/ZulNs/STM32F1_RTC
It allows to use the RTC functionalities of STM32F1xx based boards using external low speed oscillator (LSE). and keep continuing the counting after VDD is out with attached RTC battery.
I just want to inform you that maybe it save you times and use it's code.