CARP issue with one single VLAN - backup on both units

[Mistery]

Hi all, I am experiencing an issue with a single CARP interface shown in backup status on both master and backup units. My HA setup consists of a primary unit and a backup unit, with multiple interfaces (WAN, LAN and VLANs on LAN interface). HA is correctly configured and it's working fine for all VIPs except for a single CARP interface for a recently added VLAN, this VIP is shown in backup status on both units.
I tried everything I could to get the issue solved, tried entering maintenance mode, disabling CARP, rebooting both units, removing and adding the CARP interface again.
I have checked the VHID and it's correct on both units, every single interface has a unique VHID assigned.
Does anyone know what could be causing such issue ?

[Mistery]

Just want to add the following.

In dmesg log of primary unit the following log lines are shown

ifa_maintain_loopback_route: deletion failed for interface igb0_vlan6: 3
carp: 5@igb0_vlan6: BACKUP -> MASTER (master timed out)
carp: 5@igb0_vlan6: MASTER -> BACKUP (more frequent advertisement received)

Both master and backup carp status
# sysctl -a | grep "net.inet.carp."
net.inet.carp.ifdown_demotion_factor: 240
net.inet.carp.senderr_demotion_factor: 240
net.inet.carp.demotion: 0
net.inet.carp.log: 1
net.inet.carp.preempt: 0
net.inet.carp.allow: 1

[epoch]

I was playing w/ CARP yesterday and saw just that on a VLAN interface, except in my case both router instances thought they were master.

That was using an iffy virtualbox network, I thought VLAN was not supported somehow, but apparently the problem might not be with VB.

The interface itself worked ok and the sync link as well, no issue with other  "native" interfaces.