Hi, Can you please modify ebuilds to install 3rd party fish completions to /usr/share/fish/vendor_completions.d/ that is intended dir for packages to install completions into. /usr/share/fish/completions is meant only for fish's own bundled completions.
fixed in https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=443fe58c757593190412da83098e9d118c63028d