The setup is the following: gentoo prefix on a OpenSuse host. when running etc-update to manage updates to the configuration files, the HOST OS information is used to define the get_basename, get_basename_find_opt, get_scan_regexp, get_live_file sh functions. Those search for rpmsave, rpmorig, and rpmnew patterns. Hence, it does not detect the pending cfg updates.
Oh... in Prefix it shouldn't try to detect anything, it should only do Gentoo-style stuff indeed!
does it work correctly for you if you change etc-update OS_RELEASE_ID line to read OS_RELEASE_ID=$(cat "/path/to/your/prefix"/etc/os-release 2>/dev/null | grep '^ID=' | cut -d'=' -f2 | sed -e 's/"//g') ?
The fix is in portage-2.3.45
