On a CentOS system, a dirty but workable kludge would be to make /etc/resolv.conf immutable while the file is in the desired state. I think (but am not sure) that it would work for Ubuntu, as well.
It’s certainly not the preferred way of doing things – disabling or reconfiguring whatever is hosing the configuration would be the right way – but it should work as a workaround while you’re figuring that out.
chattr +i /etc/resolv.conf
would prevent the file from being changed, even by root. If root ever needs to change it, then
chattr -i /etc/resolv.conf
would have to be run first.
Richard