IPv6 forwarding enabled by default on one interface

[Compizfox]

I was pulling my hair out on why one of my interfaces on a fresh Proxmox VE 7.2-1 install did not get a IPv6 SLAAC address, until I found out that IPv6 forwarding was enabled for that one interface. E.g.:

root@eos:~# cat /proc/sys/net/ipv6/conf/vmbr0/forwarding                                                                              
1

After setting this to 0 (as it should be), IPv6 SLAAC was working as expected.

Now, why (and where) is this set to 1 by default, and where can I turn off this behavior?

For reference, the relevant part in my /etc/network/interfaces is:

auto vmbr0
iface vmbr0 inet static
        address 192.168.4.2/24
        gateway 192.168.4.1
        bridge-ports enp4s0.1 enp4s0.5
        bridge-stp off
        bridge-fd 0

 

[Compizfox]

Every reboot, IPv6 forwarding for this interface gets enabled again.

Can anyone tell me where this is being set from? Both /etc/sysctl.conf and /etc/sysctl.d/99-sysctl.conf are empty (well, only comments).