@PhoneBoy is correct, what I have seen, are issues, when "Get Interfaces with Topology" is selected overwriting any existing Anti Spoofing settings.
To avoid any issues with already defined interfaces, you should add the interface to each GW in the cluster and then "Get Interfaces without Topology" and define any Anti Spoofing you desire manually.
Example below is using existing interface eth0 and VLAN ID 200 with subnet 192.168.200.0/24 and assuming this is a topology defined by IP/Subnet
GW1
add interface eth0 vlan 200
set interface eth0.200 ipv4-address 192.168.200.2 mask-length 24
save config
GW2
add interface eth0 vlan 200
set interface eth0.200 ipv4-address 192.168.200.3 mask-length 24
save config
SmartConsole
Open cluster object, select "Network Management"
Drop down "Get Interfaces" and select "Get Interfaces without Topology"
Define your new interface Network Type (Cluster) and cluster IP address (192.168.200.1)
Select Modify under "Topology" and define network accordingly (either Network is defined by interface IP and NetMask, or an Anti Spoofing Group as needed)
Publish and Install Policy
On GW cphaprob -a if (should now show the new interface and cluster address)