preempt not working, master not receiving vrrp advertisements

Keepalived version 1.2.2 (I know, ancient, will try to update to 1.2.22
next).
Environment: VyOS 64-bit based on Debian 8, with 4.4.5 kernel
vmac mode
Computer system: same behavior in VMware VM and Dell server.

keepalived-1.2.2 preempt works fine as far back as 2012 and beyond,
and still works fine in our 32-bit Debian 6 systems.

But we need to migrate to VyOS 64-bit Debian 8. That's when we
notice preempt stop working.

Not sure if firewall rule is needed. I added one just in case:
> iptables -I INPUT -d 224.0.0.0/8 -p vrrp -j ACCEPT

To debug this problem, I added log_message in vrrp_scheduler.c
vrrp_read_dispatcher_thread() function to tell me timeout or have packet to
read.
When the low priority node is the backup, the outcome is have packet to read
every 4 s (my advert interval). Once the low priority node becomes master,
every 4 s timeout occurs, even after the high priority node comes back on
line
and both think they are master, and both sending out advertisements.

Other things checked:
- tcpdump on both nodes shows adverts from both
- "iptables -t raw -nvL" don't have rules at all, just some chains about
conntrack.

Any suggestions what may cause the master to not able to receive adverts?
What else can I do to debug this?

Thanks for help.
Siaj

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