If your system hibernated during VPN session - always assume your VPN connection is broken. To avoid re-connection issues and DNS leaks - disconnect and reconnect your VPN session before resuming internet activities.
NordVPN is reported having these issues on Arch based distributions.
The firewall issue can be solved by using firewalld which is a nftables based firewall.
One of the issues with NordVPN I have been able reproduce is the issue of hibernation.
When resuming from hibernation there is no resolvers defined in /etc/resolv.conf and you need to disconnect/reconnect to NordVPN.
From the sources below it appears that NordVPN assumes the resolver to systemd-resolvd so systems using openresolv suffers.
It is not an entirely bad suffering as it breaks your connection making it impossible to connect to access the internet - you could call it a killswitch.