/etc/init.d/networking start does not bring up interfaces configured with source-directory

Busybox supports source-directory since version 1.30.0

e.g.

auto lo
iface lo inet loopback

source-directory /etc/network/interfaces.d

i.e. if eth0 is configured in /etc/network/interfaces.d/eth0, ifup eth0 works fine.

The problem is that the rc script /etc/init.d/networking does not bring up any interfaces found via source-directory

The problematic code is this

# find interfaces we want to start
find_ifaces() {
        if [ -n "$single_iface" ]; then
                echo $single_iface
        else
                awk '$1 == "auto" {for (i = 2; i <= NF; i = i + 1) printf("%s ", $i)}' "$cfgfile"
        fi
}

i.e. it only looks for interfaces in $cfgfile (/etc/network/interfaces)

Any reason that we do not simply call ifup -a in the rc script?