Tested with Arduino IDE 1.8.13 and also Arduino IDE 1.9.0-beta for both STM32F103C8T6 and STM32F401CCU. In an issue for that library the author mentions that the core should be setting the flag ...

The newer Arduino_Core_STM32 version of Arduino support is from the ST company and is based on ST's HAL code (not on the earlier handcrafted libmaple). There is currently no available flash-loaded ...