Packages not found on x86_64 preseed against 64-bits only mirror

I'm having an issue with preseed that my install cannot find the
freeipa-client package when I use my own local mirror which only
contains the 64-bits packages synced using debmirror.

I have tried to set:

d-i apt-setup/local0/repository string [arch=amd64]
http://local.server/ubuntu xenial main

But that doesn't work.

When I manually add [arch=amd64] in all deb lines like deb
[arch=amd64] after the install has finished and I do an apt-get update
and I search for freeipa-client the package is found on my local
mirror.

When I use a public mirror, which also contains the 32-bits packages, the installation goes well.

What is wrong here ?