Does it help if you run repoclosure command with --forcearch switch?
Like: $ dnf repoclosure --forcearch aarch64

Marek,

I implemented the forcearch setting in my rewrite of spam-o-matic using DNF via its config object info, and it still didn't work.

See: https://pagure.io/releng/pull-request/7407

Is there a way how can I reproduce and test this bug? I suppose, I will need to download this aarch64 repo, won't I?
https://kojipkgs.fedoraproject.org/compose/branched/latest-Fedora-28/compose/Everything/aarch64/os/
Can I see how exactly do you run repoclosure command?

Marek,

Pungi is using https://pagure.io/pungi/blob/master/f/pungi/wrappers/repoclosure.py to generate the repoclosure.

I tried your command with one little change - I replaced --arch with --forcearch (see  http://dnf-plugins-core.readthedocs.io/en/latest/repoclosure.html):

$ dnf repoclosure --forcearch=aarch64 --repofrompath=r,https://kojipkgs.fedoraproject.org/compose/branched/latest-Fedora-28/compose/Everything/aarch64/os/ --repo=r --check=r

The results I got seemed to be pretty reasonable. There is only one catch here: --forcearch option can not be used more than once, so you can check only one architecture a time. Is this viable for you?

Btw, I'm working on making "dnf repoclosure" more compatible with the yum. Basically I'm thinking about removing --check option and returning --lookaside option with the same meaning as in the yum. 
https://github.com/rpm-software-management/dnf-plugins-core/pull/255

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