Opened JTAC case 2019-0923-0593 and provided them with logs and RSI (during/after outage).
From the faulty device's logs:
Sep 20 23:55:01 asw2-d-eqiad /usr/sbin/cron: (root) CMD ( /usr/libexec/atrun) <-- last log, routine log Sep 21 01:28:35 asw2-d-eqiad eventd: SYSTEM_OPERATIONAL: System is operational <-- first bootup log [...] Sep 21 01:28:35 asw2-d-eqiad /kernel: savecore: Reboot reason(s): 0x1: power cycle/failure
Other members only have failing keepalive and failover logs.
Asked JTAC what happened and if we should replace the device (risks of happening again).