<keymap><global><keyboard><key id="196">shutdown</key></keyboard></global></keymap>
I spent days trying to figure out why my NUCs refused to power off with the Harmony remotes after upgrading to Nexus, I went through setting after setting on the NUCs, the BIOS ... made no sense that they'd power off under Leia and Matrix but not Nexus.
Glad I finally dug through here and found this thread. I created two files in the keymaps folder, keymaps.xml and remote.xml with this mapping and all NUCs are behaving as expected, at last!
Thank you LibreElec community.