It seems to have been fixed in the masked 2.36* but not for 2.34 which is currently unmasked.
This was "fixed" in 2.36.3 by always installing in the new directory - making it incompatible with <=bash-completion-2.1 Ideally, we want to support both directories in one ebuild, because the stabilization schedules for glib and bash-completion are unlikely to align.
That's what bash-completion-r1 eclass is for. I think $(get_bashcompdir) should suffice your needs.
It's handled in latest 2.36.x (that is now unmasked)