How and why does NetworkManager during systemd boot create a connection in /run/NetworkManager/system-connections colliding with /etc/…?

Ubuntu 19.04, 5.0.0-32-generic x86_64. My (geriatric, I know) onboard atl1 ethernet chip is getting extremely hot, even temporarily fails (until reboot), when heavily networking at 1000 MB/s. Tried to reduce speed to 100 MB/s with both the nm- app and nmcli which worked and was saved to /etc/NetworkManager/system-connections/netplan-enp3s0.nmconnection (amongst other files in there). On next…

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