Device SOS (usage_fault) when downgrading firmware

Device on firmware targeting OS 2.1.0 breathing cyan. Altered device group to one which has a released firmware targeting 2.0.1. iOTA flashed app firmware, device SOS’d.

We were forced to particle update && particle flash --usb tinker to get it back into a known state and then particle flash --usb the system parts in reverse order to ensure the device was on 2.0.1 before flashing the firmware we wanted.

We’re assuming that this is just because the lower-versioned firmware is untested against the higher-versioned OS, but we weren’t really expecting it.

Why ? get to DFU mode flash all step by step as in doc.

Do you have any functions which is not void (should return something but didn’t ) ?