If you have a local e.g. Wi-Fi router and devices connected to it, e.g. smartphones, you will definitely see some local "leaked" traffic except it's not leaking, it's just local and it should belong to your local net, e.g. 192.168.0.0/24. Your ISP shouldn't see this traffic as it shouldn't leave the confines of your LAN.
VPN leaking primarily concerns your DNS requests - in order to avoid this you have several options:
- Set-up DoT (DNS over TLS) for your DNS resolver - this can be trivially done for systemd-resolved. Do not use network/DHCP provided DNS servers in this case. For Network Manager that would be "IPv4 Settings" -> Method -> "Automatic DHCP addresses only" and set DNS servers to
- Enable DoH (DNS over HTTPS) for your web browser - Firefox has this in "Network Settings" -> "Configure how Firefox connects to the internet" -> "Enable DNS over HTTPS". For Google Chrome that would be chrome://settings/security -> "Use Secure DNS".