Does High Sierra erase FileVault key on hibernate?
Apparently removing /Library/Preferences/com.apple.PowerManagement.plist, and then running an pmset -a DestroyFVKeyOnStandby 1 as Root has gotten things working reliably again. 

I have no idea what is causing this file to take precedence over the com.apple.PowerManagement.&lt;UUID&gt;.plist file again on every update, but it is rather annoying to have to remember to remove the non-UUID config file every time.