Just wanted to follow up on my last post.
We found root cause on our PCB – a digital input circuit with a strong pullup was backfeeding power to the Boron during EN power cycles, which caused it to get stuck stuck in this state. Our fix for this was to disable the pullup before power cycling using EN.
In my opinion, Particle should make it clear in the Boron datasheet that the EN pin has this problem and provide some simple suggestions on fixing it. Would have been much nicer to know about this before designing a PCB for the Boron.