This makes it possible to sync repositories inside $ROOT.
There's a patch in the following branch: https://github.com/zmedico/portage/tree/bug_563836 I've posted it for review here: https://archives.gentoo.org/gentoo-portage-dev/message/f2b7fbf4f426c3adc1ca454463054f90
This is in the master branch: https://gitweb.gentoo.org/proj/portage.git/commit/?id=28aaa75494f32c7a8b57d36c226dd37f3b9dcff9
Fixed in 2.2.24.