What’s going on right now is a very long, laborious process of rebuilding every package with a dependency on xz/lzma.
More detail on that here.
Here’s a little snippet from #ubuntu-release on IRC yesterday that should explain more the particular behavior you’re seeing:
< vorlon> so here's what's happening now
< vorlon> we're still finalizing the list of packages that would need rebuilt, but right now we have 7207 packages whose current amd64
binaries were built after xz-utils publication
< vorlon> I'm in the process of copying old, good versions of these packages (one by one) to noble-updates
< vorlon> as they publish, I will be removing the current amd64 binaries from noble+noble-proposed
< vorlon> that is slowest, because launchpad doesn't like binary removals
< vorlon> and only once that's all done we will start uploading no-change rebuilds