CoreELEC:~ # ls -al /opt/etc/passwd
lrwxrwxrwx 1 root root 11 Feb 8 09:01 /opt/etc/passwd -> /etc/passwd
Which means entware is using passwd file from CoreELEC where those files are readonly.
But you can try to remove symbolic links and copy files from CE. Maybe it will work (do it on your own risk).
rm /opt/etc/passwd /opt/etc/shadow /opt/etc/group
cp /etc/passwd /etc/shadow /etc/group /opt/etc