When the emerge --autounmask-continue causes the configuration to be reloaded, it makes emerge --getbinpkg fail because the freshly instantiated binarytree instances are not populated with remote metadata. I will post a patch that solves the problem.
Patch posted for review: https://archives.gentoo.org/gentoo-portage-dev/message/3ee5057d97fba8304c75a4cf225c803e https://github.com/gentoo/portage/pull/152
This is in the master branch: https://gitweb.gentoo.org/proj/portage.git/commit/?id=f479250c9cb9d82af4d621aa008d4d1e37a28a39
Fixed in 2.3.6.