I have tried to reproduce the issue but in vain
Is your application running from RAM ?
>> if so you need to change your boot pin(s) connection(s)
>> else could you monitor the NRST pin level via Oscilo or simple voltmeter ?
any way, you can launch your application in standalone mode, then try to connect to your mcu without reset (hotplug mode) using st-link utility, then read your program counter register. That will give an idea of what is going.