Aaand this crap just happened again. Suspended for less than 4 hours and the damned thing shutdown at some point. Here's the related journalctl entries..
Jan 24 14:24:40 Vostok NetworkManager[1354]: <info> [1516829080.5343] manager: sleep requested (sleeping: no enabled: yes)
Jan 24 14:24:40 Vostok NetworkManager[1354]: <info> [1516829080.5347] manager: sleeping...
Jan 24 14:24:40 Vostok NetworkManager[1354]: <info> [1516829080.5368] device (wlp2s0): state change: activated -> deactivating (reason 'sleeping', internal state 'managed')
Jan 24 14:24:40 Vostok NetworkManager[1354]: <info> [1516829080.5468] device (wlp2s0): state change: deactivating -> disconnected (reason 'sleeping', internal state 'managed')
Jan 24 14:24:40 Vostok NetworkManager[1354]: <info> [1516829080.6205] device (wlp2s0): state change: disconnected -> unmanaged (reason 'sleeping', internal state 'managed')
Jan 24 14:24:41 Vostok systemd-sleep[30771]: Suspending system...
Jan 24 14:24:54 Vostok kernel: ACPI: Preparing to enter system sleep state S3
Jan 24 14:24:54 Vostok kernel: cache: parent cpu1 should not be sleeping
Jan 24 14:24:54 Vostok kernel: cache: parent cpu2 should not be sleeping
Jan 24 14:24:54 Vostok kernel: cache: parent cpu3 should not be sleeping
Jan 24 14:24:54 Vostok kernel: cache: parent cpu4 should not be sleeping
Jan 24 14:24:54 Vostok kernel: cache: parent cpu5 should not be sleeping
Jan 24 14:24:54 Vostok kernel: cache: parent cpu6 should not be sleeping
Jan 24 14:24:54 Vostok kernel: cache: parent cpu7 should not be sleeping
Jan 24 14:24:54 Vostok kernel: ACPI: Waking up from system sleep state S3
Jan 24 14:24:54 Vostok systemd-sleep[30771]: System resumed.
Jan 24 14:24:54 Vostok systemd[1]: sleep.target: Unit not needed anymore. Stopping.
Jan 24 14:24:54 Vostok systemd-logind[1313]: Operation 'sleep' finished.
Jan 24 14:24:54 Vostok NetworkManager[1354]: <info> [1516829094.2648] manager: wake requested (sleeping: yes enabled: yes)
Jan 24 17:19:54 Vostok NetworkManager[1354]: <info> [1516839594.3900] manager: sleep requested (sleeping: no enabled: yes)
Jan 24 17:19:54 Vostok NetworkManager[1354]: <info> [1516839594.3907] manager: sleeping...
Jan 24 17:19:54 Vostok NetworkManager[1354]: <info> [1516839594.3938] device (wlp2s0): state change: activated -> deactivating (reason 'sleeping', internal state 'managed')
Jan 24 17:19:54 Vostok NetworkManager[1354]: <info> [1516839594.4077] device (wlp2s0): state change: deactivating -> disconnected (reason 'sleeping', internal state 'managed')
Jan 24 17:19:54 Vostok NetworkManager[1354]: <info> [1516839594.4957] device (wlp2s0): state change: disconnected -> unmanaged (reason 'sleeping', internal state 'managed')
Jan 24 17:19:55 Vostok systemd-sleep[28758]: Suspending system...