I filed https://pagure.io/releng/issue/7071 about an issue with the creation with composes. Their contents is no longer predictable, and multilib packages appear and disappear without any apparent cause.
I think this has to be a release blocker. F27 seems to be much more prominently affected than F26, although we have seen at least one report with F26 updates-testing as well.
I don't think this is related to glibc. Downstream, we see similar issues with glibc first because necessarily it's part of any multilib installation. (And downstream, it's always a local repository misconfiguration, and not an issue with the composes.) On the other hand, I don't have a RCA for the F27/F26 issues, so maybe we are doing something unusual which triggers the non-determinism.
Practically speaking, only Bugzilla bugs can reliably be handled as release blockers. The release blocker tracking mechanism only really works with Bugzilla bugs. So we will need someone to create a bug in Bugzilla (I'd suggest 'distribution' as the component, it's the one I tend to use as a catch-all for issues of this nature which can't yet be related to a specific package) and propose it as release-blocking (use the blockerbugs webapp, or just mark the bug as blocking FinalBlocker).
FinalBlocker
The issue referenced as I mentioned to @fweimer in the releng issue he mentioned is a duplicate of https://pagure.io/releng/issue/4084 that is 7 years old. it is also an issue only effecting updates and updates-testing and until raised by @fweimer we had not had a single report of it affecting people for many years. I strongly believe its not a blocker. It is not affecting any composes, only the updates process as a result there is nothing that we can do in the release process to fix it.
On that basis I'd agree it should not be a release blocker.
Metadata Update from @sgallagh: - Issue tagged with: meeting
AGREED: FESCo does not consider this blocking for the release (0, 1, -5) (sgallagh, 16:58:49)
Metadata Update from @sgallagh: - Issue close_status updated to: Fixed