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 ...

To Upload the code, before push the button to Load code in ARDUINO IDE, you must put the BluePill board in "native bootoader" mode. To active this mode, you have to change the jumpers positions: ...