I'm really not sure if this is a bug or by design. I have a system which I use a more powerful system to build binaries for in a chroot. When updating the target, the package libarchive triggered a 'r' (rebuild) on kde-base/ark as it should. However it didn't use the binary from the binhost, it wanted to rebuild from source on the target. Reproducible: Always
There are many factors that can cause results to vary. Here are some common cases: 1) If the target system has different USE settings, then you can use --binpkg-respect-use=n to make it try to choose the binary package even when building it from source would result in different USE settings. 2) If the binary package was built different KEYWORDS that the ebuild currently has, and the binary package is masked by KEYWORDS on the target system, then you may need to adjust /etc/portage/package.accept_keywords on the target system. 3) If the binary package corresponds to an ebuild version which is no longer available in the current portage tree, then in will be rejected in favor of an ebuild which is currently available. There may be other reasons that I've forgotten to mention. The --usepkgonly option can help for cases (1) and (3).
Thanks Zac for the very exact response :) It seems it was probably just a combination of #2 and poor timing. When I built/installed the binpackages last week kde-base/ark was in ~x86. Since then they had been stabled, and I didn't do a full bin rebuild. Rebuilding ark on the server fixed it up and explains it perfectly. Just wanted to make sure I didn't accidentally discover a bug. Closing.