Kernel IP forwarding not enabled for ethernet interface on boot

It turns out it is a bug in ifupdown2 utility, which I was unaware was even installed. Until bug resolution gets into Buster, the workaround is to set ip-forward parameter in /etc/network/interfaces:

iface enp0s25 inet static
    address x.x.x.x
    netmask 255.255.255.0
    gateway x.x.x.x
    ip-forward 1

Kernel IP forwarding not enabled for ethernet interface on boot

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