I'm having this problem too with Ubuntu 19.10. I was able to workaround by commenting out "primary: enp65s0", but this solution is far from optimal because eno1 is 10 GbE and enp65s0 is 56 GbE. I always want enp65s0 to be the primary, unless it is unavailable.

root@lab02:~# cat /etc/netplan/01-netcfg.yaml
network:
  version: 2
  renderer: networkd

  ethernets:
    eno1: {}
    eno2: {}
    eno3: {}
    eno4: {}
    enp65s0: {}
    dummy1: {}
    dummy2: {}
    dummy3: {}
    dummy4: {}
    dummy5: {}
    dummy6: {}

  bonds:
    bond0:
      interfaces: [eno1, enp65s0]
      parameters:
        #primary: enp65s0
        mode: balance-tlb
        mii-monitor-interval: 100
        transmit-hash-policy: layer3+4
        primary-reselect-policy: better

  vlans:
    vbridge2-vlan10:
      id: 10
      link: vbridge2
      addresses: [10.11.1.2/24]
    vbridge2-vlan20:
      id: 20
      link: vbridge2
      addresses: [10.11.2.2/24]
    vbridge4-vlan10:
      id: 10
      link: vbridge4
      addresses: [10.12.1.2/24]
    vbridge4-vlan20:
      id: 20
      link: vbridge4
      addresses: [10.12.2.2/24]
    vbridge6-vlan10:
      id: 10
      link: vbridge6
      addresses: [10.13.1.2/24]
    vbridge6-vlan20:
      id: 20
      link: vbridge6
      addresses: [10.13.2.2/24]

  bridges:
    internet:
      interfaces: [bond0]
      dhcp4: yes
      nameservers:
        addresses: [8.8.8.8, 1.1.1.1]
    vbridge1:
      interfaces: [dummy1]
      addresses: [10.11.0.2/24]
    vbridge2:
      interfaces: [dummy2]
    vbridge3:
      interfaces: [dummy3]
      addresses: [10.12.0.2/24]
    vbridge4:
      interfaces: [dummy4]
    vbridge5:
      interfaces: [dummy5]
      addresses: [10.13.0.2/24]
    vbridge6:
      interfaces: [dummy6]

root@lab02:~# cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=19.10
DISTRIB_CODENAME=eoan
DISTRIB_DESCRIPTION="Ubuntu 19.10"

root@lab02:~# ethtool eno1
Settings for eno1:
	Supported ports: [ FIBRE ]
	Supported link modes:   1000baseT/Full
	                        10000baseT/Full
	Supported pause frame use: Symmetric Receive-only
	Supports auto-negotiation: No
	Supported FEC modes: Not reported
	Advertised link modes:  10000baseT/Full
	Advertised pause frame use: No
	Advertised auto-negotiation: No
	Advertised FEC modes: Not reported
	Speed: 10000Mb/s
	Duplex: Full
	Port: Direct Attach Copper
	PHYAD: 1
	Transceiver: internal
	Auto-negotiation: off
	Supports Wake-on: g
	Wake-on: d
	Current message level: 0x00000000 (0)

	Link detected: yes

root@lab02:~# ethtool enp65s0
Settings for enp65s0:
	Supported ports: [ FIBRE ]
	Supported link modes:   1000baseKX/Full
	                        10000baseKX4/Full
	                        10000baseKR/Full
	                        40000baseCR4/Full
	                        40000baseSR4/Full
	                        56000baseCR4/Full
	                        56000baseSR4/Full
	Supported pause frame use: Symmetric Receive-only
	Supports auto-negotiation: Yes
	Supported FEC modes: Not reported
	Advertised link modes:  1000baseKX/Full
	                        10000baseKX4/Full
	                        10000baseKR/Full
	                        40000baseCR4/Full
	                        40000baseSR4/Full
	Advertised pause frame use: Symmetric
	Advertised auto-negotiation: Yes
	Advertised FEC modes: Not reported
	Link partner advertised link modes:  56000baseCR4/Full
	Link partner advertised pause frame use: No
	Link partner advertised auto-negotiation: Yes
	Link partner advertised FEC modes: Not reported
	Speed: 56000Mb/s
	Duplex: Full
	Port: None
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	Supports Wake-on: d
	Wake-on: d
	Current message level: 0x00000014 (20)
			       link ifdown
	Link detected: yes