STM32F072C8T7 needs hardware reset upon power on to work!
The STM32F072C8T7 has inbuilt POR (power on reset) capability. 
The datasheet notes (page 14):


The device has integrated power-on reset (POR) and power-down reset (PDR) circuits. They are always active, and ensure proper operation above a threshold of 2 V. The device remains in reset mode when the monitored supply voltage is below a specified threshold,VPOR/PDR, without the need for an external reset circuit.


But, is this unconditionally the case?
No, unfortunately.


The POR monitors only the VDD supply voltage.
During the startup phase it is required that VDDA should arrive first and be greater than or equal to VDD.  The PDR monitors both the VDD and VDDA supply voltages, however the VDDA power
supply supervisor can be disabled (by programming a dedicated Option bit) to reduce
the power consumption if the application design ensures that VDDA is higher than or
equal to VDD.




Page 14 also notes::

VDD = VDDIO1 = 2.0 to 3.6 V: external power supply for I/Os (VDDIO1) and the internal regulator. It is provided externally through VDD pins.

VDDA = from VDD to 3.6 V: external analog power supply for ADC, DAC, Reset blocks,
RCs and PLL (minimum voltage to be applied to VDDA is 2.4 V when the ADC or DAC
are used). It is provided externally through VDDA pin. The VDDA voltage level must be always greater or equal to the VDD voltage level and must be established first.