SUMMARY
I've got both iptables, nftables and firewalld installed. Cannot remove iptables because it is a dependency of systemd.

The issue is that the Plasma Firewall inside the System Settings shows that the "Default Incoming Policy" and "Default Outgoing Policy" are "Allow", when this is not true in practice. The currently used connection is using the wlp3s0 interface.

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Output of iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

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Output of firewall-cmd --get-active-zones
public
  interfaces: wlp3s0
trusted
  interfaces: lo

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Output of firewall-cmd --info-zone=public
public (active)
  target: DROP
  icmp-block-inversion: no
  interfaces: wlp3s0
  sources: 
  services: dhcpv6-client
  ports: 
  protocols: 
  forward: yes
  masquerade: no
  forward-ports: 
  source-ports: 
  icmp-blocks: 
  rich rules:

OBSERVED RESULT
Plasma Firewall shows that the "Default Incoming Policy" and "Default Outgoing Policy" are "Allow".

EXPECTED RESULT
Plasma Firewall should show that the "Default Incoming Policy" is "Drop" and "Default Outgoing Policy" is "Allow".

SOFTWARE/OS VERSIONS
firewalld 1.0.2-2
iptables 1:1.8.7-1
nftables 1:1.0.1-3
Operating System: Arch Linux
KDE Plasma Version: 5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2
Kernel Version: 5.15.13-zen1-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 8 × Intel® Core™ i5-8250U CPU @ 1.60GHz
Memory: 7,6 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 620