ifup cannot bring up eth1 after upgrading

This happened some hours after I upgraded to Debian Buster on my Bandwagonhost VPS. The IP timed out all over the world. Then I did a research in the root shell. It turned out to be networking.service failure:

ifup[478]: Failed to bring up eth1.
systemd[1]: networking.service: Failed with result 'exit-code'.

Searched and found a solution1:

# apt install network-manager
# systemctl enable systemd-networkd
# systemctl enable systemd-resolved
# systemctl start systemd-networkd
# systemctl start systemd-resolved
# apt install isc-dhcp-client
# dpkg-reconfigure resolvconf

Then the IP worked again, but networking.service still failed with the same error. I searched again and found it related to /etc/network/interfaces2.

My /etc/network/interfaces looked like this:

source /etc/network/interfaces.d/*
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
auto eth1
iface eth1 inet dhcp

Meanwhile the output of networkctl looked like this:

IDX LINK             TYPE               OPERATIONAL SETUP     
  1 lo               loopback           carrier     unmanaged
  2 eth0             ether              routable    unmanaged

There's nothing to do with so-called eth1, so this is my edited /etc/network/interfaces now:

source /etc/network/interfaces.d/*
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
#auto eth1
#iface eth1 inet dhcp

The reason why that happened might be due to my selection of using the config file from maintainer's package when I did the upgrade.