And why does the "flash" get erased after a power cycle?

I certainly don't know the answer to that, but Jeff just mailed me a Nucleo board, so I'll be able to look into some of these issues starting tomorrow.

Just to add more data, in my case power cycling the nucleo board does not require to do a flash again, I can power off the nucleo, put other chip to test and perform testing. I'm using a thunderbolt to USB hub, in theory that delivers more power, could be because that.