Sorry to reply to myself but just found this in the debian forums for the same issue. Perhaps it's a simple power control setting in the bsd driver that is disabling the NIC?
Message #26 received at 928328@bugs.debian.org (full text, mbox, reply):
From: "Rene 'Renne' Bartsch, B.Sc. Informatics" rene@bartschnet.de
To: 928328@bugs.debian.org
Subject: Re: Intel I350-NIC shuts down ports after loosing carrier
Date: Fri, 3 May 2019 11:01:42 +0200
echo on > /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/power/control
echo on > /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.1/power/control
echo on > /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.2/power/control
echo on > /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.3/power/control
solves the problem. This should be "on" by default instead of "auto".
Regards,
Renne