I’m not exactly sure what exactly prompts this behavior.
Could be that the bootloader part of the firmware is broken.
Maybe stm32flash and flashing via openocd write different parts of the firmware.
Did you have any errors on your output?
I had a lot of them and also tired flashing with openocd multiple times (about ~20x) until the output was a bit longer than usual … that’s when the STM finally registered with address 0x56 again.