Yup, that’s the cloudinit package deployed by OVH that’s interfering. I had tested multiple ways to block it (maybe this works for you?), but I ended up completely disabling it. You have a dedicated server, so I am afraid you would lose your network connection.
Instead, can you revert the file back to only nameserver 127.0.0.1 and then directly after let’s make it read-only: sudo chattr -i /etc/resolv.conf
Then restart the services again.
Since you are in contact with OVH support, can you ask them how to keep cloudinit enabled but prevent it from altering /etc/resolv.conf?