Ok, let me provide you with our typical configuration then - maybe it will explain a bit what I mean.

Eventually, we do not have a separate "flat" network - we provide public network through a VLAN, thus we don't have any FLAT network at all.
However, you can add a FLAT one to your configuration if you want to, it still have to be on VLAN though:
    - network:
        container_bridge: "br-public"
        container_type: "veth"
        type: "flat"
        net_name: "flat"
        network_interface: "bond0.50"
        group_binds:
          - neutron_openvswitch_agent

So our typical configuration would look like that.

openstack_user_config:

  management_bridge: br-mgmt
  tunnel_bridge: br-vxlan
  provider_networks:
    - network:
        group_binds:
          - all_containers
          - hosts
        type: raw
        container_bridge: br-mgmt
        container_interface: eth1
        container_type: veth
        ip_from_q: container
        is_container_address: true
        is_ssh_address: true
    - network:
        group_binds:
          - glance_api
          - cinder_volume
          - nova_compute
        type: raw
        container_bridge: br-storage
        container_type: veth
        container_interface: eth2
        container_mtu: 9000
        ip_from_q: storage
    - network:
        group_binds:
          - neutron_openvswitch_agent
        container_bridge: br-vxlan
        container_type: veth
        container_interface: eth10
        container_mtu: 9000
        ip_from_q: tunnel
        type: vxlan
        range: 65537:69999
        net_name: vxlan
    - network:
        group_binds:
          - neutron_openvswitch_agent
        container_bridge: br-vlan
        container_type: veth
        container_interface: eth11
        type: vlan
        range: 40:400
        net_name: vlan

And here is how netplan config looks like:
network:
    bonds:
        bond0:
            interfaces:
            - eno1
            - eno2
            macaddress: MAC
            mtu: 9000
            parameters:
                down-delay: 0
                lacp-rate: slow
                mii-monitor-interval: 100
                mode: 802.3ad
                transmit-hash-policy: layer3+4
                up-delay: 0
    bridges:
        br-mgmt:
            addresses:
            - 172.29.236.X/22
            interfaces:
            - bond0.10
            macaddress: MAC
            mtu: 1500
            parameters:
                forward-delay: 15
                stp: false
        br-storage:
            addresses:
            - 172.29.244.X/22
            interfaces:
            - bond0.20
            macaddress: MAC
            mtu: 9000
            parameters:
                forward-delay: 15
                stp: false
        br-vxlan:
            addresses:
            - 172.29.240.X/22
            interfaces:
            - bond0.30
            macaddress: MAC
            mtu: 9000
            parameters:
                forward-delay: 15
                stp: false
    ethernets:
        eno1:
            match:
                macaddress: MAC
            mtu: 9000
            set-name: eno2
        eno2:
            match:
                macaddress: MAC
            mtu: 9000
            set-name: eno2
    version: 2
    vlans:
        bond0.10:
            id: 10
            link: bond0
            mtu: 1500
        bond0.40:
            addresses:
            - <PUBLIC IP>
            gateway4: <PUBLIC IP>
            id: 40
            link: bond0
            mtu: 1500
            nameservers:
                addresses:
                - 8.8.8.8
                - 1.1.1.1
        bond0.30:
            id: 30
            link: bond0
            mtu: 9000
        bond0.20:
            id: 20
            link: bond0
            mtu: 9000