net.ipv6.conf.all.accept_ra=0 net.ipv6.conf.default.accept_ra=0 not working?!

[Reinder <reinder@reinder.nl>]

Hello Debian Users!

Maybe I am doing something silly, but I am confused, so please educate 
me if I am seeing/doing this wrong.

When trying to disable automatic binding of a (wrongly) advertised ipv6 
IP I got stuck and only got it to work with the bottom two lines below 
in my /etc/sysctl.d/am-no-ipv6.conf:

root@reinder:~# cat /etc/sysctl.d/am-no-ipv6.conf
net.ipv6.conf.all.accept_ra=0
net.ipv6.conf.all.autoconf=0
net.ipv6.conf.default.accept_ra=0
net.ipv6.conf.default.autoconf=0
net.ipv6.conf.ens18.accept_ra=0
net.ipv6.conf.ens19.accept_ra=0
root@reinder:~#

This is, to me, unexpected, even unwanted behavior?
I would expect disabling default and all to work and prevent ipv6 
routing and perhaps need of firewalling?!

What I also tried before and still have access but does not prevent an 
ipv6 from binding:

root@reinder:~# tail -7 /etc/dhcpcd.conf
## only ipv4
ipv4only
ipv6ra_noautoconf
nodhcp6
noipv6
noipv6rs
noipv4ll
root@reinder:~#

root@reinder:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug ens18
iface ens18 inet dhcp
iface ens18 inet6 static
        autoconf 0
        accept_ra 0

# The secondary network interface
allow-hotplug ens19
iface ens19 inet dhcp
iface ens19 inet6 static
        autoconf 0
        accept_ra 0
root@reinder:~#

Kind regards,

Reinder