My Photon was working fine on 5V VIN, but the TTL port that I was powering it from caused the main device to not boot up properly. That board has a 12V input.
I soldered my VIN to the 12V wire coming out of the power supply and then my Photon just started SOS for Hard Fault and the main device LCD didn’t power up.
Removed the Photon and now my device works again, but Photon is in Hard Fault. It won’t come up with yellow LED if I hold both buttons and release reset.