I believe the broken upgrade is due to a scoping issue when $INTERFACESv4 isn't defined in /etc/defaults/isc-dhcp-server ($DHCPD_DEFAULT).
To fix the upgrade process an update is needed to the /etc/init.d/isc-dhcp-server file on 4.3.4-1 to define $INTERFACESv4 before sourcing $DHCPD_DEFAULT, eg: <snip> ....... . /lib/lsb/init-functions INTERFACESv4="" # Read init script configuration [ -f "$DHCPD_DEFAULT" ] && . "$DHCPD_DEFAULT" ...... </snip>
