I've just installed the latest nightly for v12 generic (LibreELEC-Generic.x86_64-12.0-nightly-20230628-73e6606.img) and I can't alter the system name from LibreElec.
I found this thread System Name always reverts to LibreELEC after reboot from June this year and this thread Change hostname? from 2018. The recommendation in this years thread was to edit hostname which is why I found the earlier thread.
Here is my hostname (in line with chewitt's info)
<?xml version="1.0" ?>
<libreelec>
<addon_config/>
<settings>
<system>
<wizard_completed>True</wizard_completed>
<KeyboardLayout1>uk</KeyboardLayout1>
<hostname>Optiplex</hostname>
</system>
<connman>
<wizard_completed>True</wizard_completed>
</connman>
<services>
<wizard_completed>True</wizard_completed>
</services>
<about>
<wizard_completed>True</wizard_completed>
</about>
<libreelec>
<wizard_completed>True</wizard_completed>
</libreelec>
<updates>
<AutoUpdate>manual</AutoUpdate>
<SubmitStats>0</SubmitStats>
</updates>
</settings>
</libreelec>