This seems to happen every-so-often to some of my micros even in the same hardware. When my board is completely unpowered and only Uart is connected to PA18/PA21 as seen in the cut down schematic below through an FTDI cable, I then power up my board and my micro doesn't boot. Now my idea is that the low power coming through the ftdi cable that has the capability to operate the micro switches some of the peripherals on, but not all of them. That can be confirmed when I power the board up and the wdt reset kicks in...sometimes. My main question is how could I combat this? The obvious answer is to disconnect the debug cable while I am powering the system, but in terms of practically have other people seen this in the past?
I am yet to look at the brown-out detector on the M0+ series, perhaps this is the only way to do it? Else I was thinking of having to try and use an isolated debug cable.