Thanks for that... but it also got me thinking. I've only had an issue with this once when the br-lan bridge derived it's MAC from eth1 (the LAN). All other times that br-lan derives it's MAC from eth1, I don't have an issue (including now). In this case,
ip -6 addr show dev br-lan
showed the br-lan ipv6 lla as ...dadfailed tentative...
So instead of trying to change the ipv6 lla's I've just removed all of them from the r7500v2 AP interfaces except for the lo interface (details below). I still have ipv6 (but this needs a long term test as clients can cache an ipv6 address - something that has caused me grief in the past). Now, even if br-lan derives its MAC from eth0 I don't see any "own source as address" messages (a configuration that, for me, causes these messages from boot).
Again I consider this a kluge workaround and not really a solution. I still don't know what the problem is, but I am symptom free for the hour or two that I have tested. Also, I am not using STP on the bridges.
To remove the ipv6 lla's from the AP interfaces (including the wireless interfaces) I put the following script in /etc/hotplug.d/net (don't use the iface directory):
#!/bin/sh
#env > /tmp/envs_log.log
if [ "$ACTION" != "add" ]; then
exit
fi
logger="/usr/bin/logger"
scope="link"
ip -6 addr flush scope "${scope}" dev eth0
ip -6 addr flush scope "${scope}" dev eth1
ip -6 addr flush scope "${scope}" dev wlan1
ip -6 addr flush scope "${scope}" dev wlan1-1
ip -6 addr flush scope "${scope}" dev wlan0
ip -6 addr flush scope "${scope}" dev wlan0-1
ip -6 addr flush scope "${scope}" dev br-lan
ip -6 addr flush scope "${scope}" dev br-guestWLAN
echo "/etc/hotplug.d/net/99-rmlla: lla's removed" | $logger
I give the script execute permissions via
chmod ugo+x /etc/hotplug.d/net/99-rmlla