Bridge config in netplan does not come up at boot

Question

I had an ubuntu 18.04 working and decided to move it to a VM in Proxmmox.

Configuration includes a bridge, it was working fine in the previous system but it does not come up at boot in the Proxmox VM.

Here is my netplan config:

network:
    renderer: networkd
    ethernets:
            ens18:
                    dhcp4: false
                    addresses: [172.18.0.4/29]
                    gateway4: 172.18.0.1
                    nameservers:
                            addresses: [8.8.8.8]
            ens19:
                    dhcp4: false
            ens20:
                    dhcp4: false
            ens21:
                    dhcp4: false
    bridges:
            br0:
                    addresses: [10.0.0.1/16]
                    interfaces: [ens19, ens20, ens21]
                    gateway4: 10.0.0.1
                    dhcp4: false
    version: 2

When system is up I can run

netplan apply

and it works perfectly.

I have also tried as a temporal solution to use crontab to do this,

@reboot netplan apply

but it's not working either.

netplan --debug generate

generates no error.

I've seen these messages in journalctl

Jul 11 10:20:49 kernel: br0: port 3(ens19) entered disabled state
Jul 11 10:20:49 kernel: br0: port 2(ens20) entered disabled state
Jul 11 10:20:49 kernel: br0: port 1(ens21) entered disabled state
Jul 11 10:20:49 systemd-networkd[643]: br0: Lost carrier
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 kernel: device ens19 left promiscuous mode
Jul 11 10:20:49 kernel: br0: port 3(ens19) entered disabled state
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 kernel: device ens20 left promiscuous mode
Jul 11 10:20:49 kernel: br0: port 2(ens20) entered disabled state
Jul 11 10:20:49 kernel: device ens21 left promiscuous mode
Jul 11 10:20:49 kernel: br0: port 1(ens21) entered disabled state
Jul 11 10:20:49 networkd-dispatcher[785]: WARNING:Unknown index 3 seen, reloading interface list
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 networkd-dispatcher[785]: WARNING:Unknown index 4 seen, reloading interface list
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 networkd-dispatcher[785]: WARNING:Unknown index 5 seen, reloading interface list
Jul 11 10:20:49 systemd-networkd[643]: ens19: Could not join netdev: No such device
Jul 11 10:20:49 systemd-networkd[643]: ens19: Failed
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-networkd[643]: ens20: Could not join netdev: No such device
Jul 11 10:20:49 systemd-networkd[643]: ens20: Failed
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-timesyncd[657]: Network configuration changed, trying to establish connection.
Jul 11 10:20:49 systemd-networkd[643]: ens21: Could not join netdev: No such device
Jul 11 10:20:49 systemd-networkd[643]: ens21: Failed

so I suspect interfaces are not ready when trying to create the bridge.

I've been looking for a way to execute the netplan apply after these interfaces are ready but haven't find how to. I believe the same service does both things.

Why is not applying configuration at startup? I'm I loosing something? Is there an issue with Proxmox and Ubuntu bridge?