At the moment of loss of power, the Pi will not have time to write anything to a log. You will only see entries from when the power comes back and the unit boots.
The command "uptime" will tell you how long it was since last boot.
The command "journalctl --list-boots" will give you a list of boots that is in the system logs.
( If the last command says that there are no journal files, try with "sudo" and/or create the folder "/var/log/journal" )