Ok there is actually several problems here.

1. When you run the reposync with -n (newest packages only) you also download metadata, but these are metadata for the whole repository not just the newest packages. Given that repomanage needs to work with modules it uses the metadata, this basically means that it currently doesn't matter what rpms you have downloaded, if you download the repodata for the whole repository repomanage works as if you had downloaded the whole repository. For example when running with --new there is rhel-8-for-x86_64-appstream-rpms/Packages/p/perl-5.24.4-403.module+el8+2770+c759b41a.x86_64.rpm but this rpm was not downloaded (its missing in the  # find . -name perl-5* output). It is not clear to me how (if) this should be fixed. There is kind of missing createrepo step.

2. Regarding the actual output
> Expected results:
> The oldest package from each module AND, if applicable, non modular content should be shown. 
>
> ./rhel-8-for-x86_64-appstream-rpms/Packages/p/perl-5.26.3-416.el8.x86_64.rpm
> ./rhel-8-for-x86_64-appstream-rpms/Packages/p/perl-5.24.4-404.module+el8.1.0+2926+ce7246ad.x86_64.rpm
> ./rhel-8-for-x86_64-appstream-rpms/Packages/p/perl-5.30.1-451.module+el8.3.0+6961+31ca2e7a.x86_64.rpm

The --old option is probably not documented enough, but it is showing only older packages not oldest. Meaning if there is only one package in a stream it is not older its the newest package and it doesn't get shown. However there is a bug, it should not be showing rhel-8-for-x86_64-appstream-rpms/Packages/p/perl-5.24.4-404.module+el8.1.0+2926+ce7246ad.x86_64.rpm but instead rhel-8-for-x86_64-appstream-rpms/Packages/p/perl-5.24.4-403.module+el8+2770+c759b41a.x86_64.rpm.

This should be addressed in a PR: https://github.com/rpm-software-management/dnf-plugins-core/pull/414 (together with another caching issue that may be causing problems on multiple consecutive runs of repomanage)

3. Finally I would like to note that there is a general problem of how to figure out the package order in a stream, it is not easy to tell which packages are newest and which are older. Repomanage with the mentioned PR fix is showing rpms from the latest versions of a module streams but these might not be the rpms with the latest nevras. It is a similar issue to this reposync bz: https://bugzilla.redhat.com/show_bug.cgi?id=1833074

@amatej
I just noticed that the rebase from https://bugzilla.redhat.com/show_bug.cgi?id=1951414 is pulling in the upstream PR you mentioned in https://bugzilla.redhat.com/show_bug.cgi?id=1898293#c1. That should cover the majority of the issues here.

Want to track this as resolved via that rebase?

I wasn't sure it sufficiently fixes the customer issue but looking at this now I made another PR: https://github.com/rpm-software-management/dnf-plugins-core/pull/431 that enhances repomanage documentation to better reflect the actual behavior (it also allows running repomanage with just repodata). 

I would prefer if we also backported this new PR to resolve this, otherwise it seems kind of incomplete to me.

My new PR was merged.

Linking also some older test updates related to this: https://github.com/rpm-software-management/ci-dnf-stack/pull/913

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: dnf security and bug fix update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2021:4464