Adding the following line to /etc/portage/package.use works around the bug: =dev-python/flask-2.0.1 -doc
Fixed in version 2.0.1-r1, in the following commit https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=49d0750307ebbce63fd32ad0c0580c71bf22461a Thank you for the report