Looking at the netconsole-setup shell script, the problem arises when
the kernel doesn't know (yet) about the destination address.
If you add a ping before the neighbour lookup it'll probably work.
So:
--- /usr/sbin/netconsole-setup 2024-06-22 08:56:28.000000000 +1000
+++ /var/tmp/netconsole-setup 2024-07-03 08:46:43.255865757 +1000
@@ -35,6 +35,7 @@
local address="$1"
local from_interface="$2"
+ /usr/bin/ping -c 1 "$address" > /dev/null 2>&1
mac=$(ip -o neigh show to "$address" dev "$from_interface" | grep -oE
"lladdr [^ ]*" | cut -d ' ' -f 2)
if test -z "$mac"; then
# Run arping to determine the MAC address for the given IP address
