I think that the wrong "Bug-Debian" tag was inserted in debian/patches/protobuf_generated_classes_no_inheritance.patch:
Description: Fix build with latest protobuf Origin: gentoo https://gitweb.gentoo.org/repo/gentoo.git/plain/app-i18n/mozc/files/mozc-2.23.2815.102-protobuf_generated_classes_no_inheritance.patch Bug-Debian: http://bugs.debian.org/265678
