Boot0

All STM32 MCUs includes a GPIO pin labelled Boot0. This pin is used to select the boot mode

Hardware design

It is extremely important to pull the Boot0 pin low during reset for normal operation.

If the pin is left unconnected in a hardware design it will "probably work", but it "might" on occasion detect a "high" on the pin and therefore decide to switch to the internal bootloader.

The proper way to deal with it can be seen on my Green Pill design:

The button SW2 is of course optional but the pull-down resistor R5 is not.