Hi,

Did some more test, I changed the network daemon from systemd-networkd to NetworkManager (still use netplan), the 'vlan interface lost after reboot' still there: reboot the os will lost the vlan interface, restart the NM service could shows up the vlan interface. In the NM log, it shows during the boot stage: manager: "(netplan-enc1000.1292) couldn't create the device: no support for VLANs on interface enc1000 of type Ethernet"

Here are some networkmanager service command output before and after 'restart nm service' as the comparison, and journalctl log file with operate timestamps:
>>>>>>>>>>>
journalctl -u NetworkManager > nm.log

timestamp: Jul 27 08:27 -> reboot the ubut os

timestamp: Jul 27 08:32 -> restart the nm service
>>>>>>>>>>>>

>>>>>>>>>>>>>>>>>>>
run 'nmcli d' and 'nmcli c' before and after 'restart nm'

> before NM restart 

ubuntu@a90ubut15:~$ nmcli d                                                     
DEVICE   TYPE      STATE                   CONNECTION                           
enc1000  ethernet  connected               netplan-enc1000                      
lo       loopback  connected (externally)  lo                                   

ubuntu@a90ubut15:~$ nmcli c                                                     
NAME                  UUID                                  TYPE      DEVICE    
lo                    2f1c6dd9-26ca-4276-a210-8bf61256322a  loopback  lo        
netplan-enc1000       20393913-b9a9-3c3a-81cb-20f5840b309b  ethernet  enc1000   
netplan-enc1000.1292  491497ea-36cb-3b96-ad99-6519b363483d  vlan      --        

> after restart NM

ubuntu@a90ubut15:~$ nmcli d
DEVICE        TYPE      STATE                   CONNECTION
enc1000.1292  vlan      connected               netplan-enc1000.1292
enc1000       ethernet  connected               netplan-enc1000
lo            loopback  connected (externally)  lo

ubuntu@a90ubut15:~$ nmcli c
NAME                  UUID                                  TYPE      DEVICE
netplan-enc1000.1292  491497ea-36cb-3b96-ad99-6519b363483d  vlan      enc1000.1292
lo                    2f1c6dd9-26ca-4276-a210-8bf61256322a  loopback  lo
netplan-enc1000       20393913-b9a9-3c3a-81cb-20f5840b309b  ethernet  enc1000
>>>>>>>>>>>>>>>>>>>>>>>>>



>>>>>>>>>>>>>>>>>>>>>>>>>>
I also compared the output of 'nmcli c show netplan-enc1000.1292' command before and after 'restart nm', found that the output of 'after the restart nm' not only include all the parameters in 'before the restart nm', and also plus the following parameters:

GENERAL.NAME:                           netplan-enc1000.1292
GENERAL.UUID:                           491497ea-36cb-3b96-ad99-6519b363483d
GENERAL.DEVICES:                        enc1000.1292
GENERAL.IP-IFACE:                       enc1000.1292
GENERAL.STATE:                          activated
GENERAL.DEFAULT:                        yes
GENERAL.DEFAULT6:                       no
GENERAL.SPEC-OBJECT:                    --
GENERAL.VPN:                            no
GENERAL.DBUS-PATH:                      /org/freedesktop/NetworkManager/ActiveConnection/3
GENERAL.CON-PATH:                       /org/freedesktop/NetworkManager/Settings/2
GENERAL.ZONE:                           --
GENERAL.MASTER-PATH:                    --
IP4.ADDRESS[1]:                         10.20.92.201/24
IP4.GATEWAY:                            10.20.92.254
IP4.ROUTE[1]:                           dst = 10.20.92.0/24, nh = 0.0.0.0, mt = 400
IP4.ROUTE[2]:                           dst = 0.0.0.0/0, nh = 10.20.92.254, mt = 400
IP4.DNS[1]:                             10.20.0.2
IP6.ADDRESS[1]:                         fe80::ca:72ff:fe80:acb0/64
IP6.GATEWAY:                            --
IP6.ROUTE[1]:                           dst = fe80::/64, nh = ::, mt = 256
>>>>>>>>>>>>>>>>>>>>>>>>>>

Thanks.