It gets installed as /usr/share/bash-completion/completions/pass It should be /usr/share/bash-completion/pass
And in case it wasn't clear, the consequence is that eselect bashcomp does not recognize it.
--- ./ChangeLog +++ ./ChangeLog @@ -4,0 +5,4 @@ + 08 May 2014; Jason A. Donenfeld <zx2c4@gentoo.org> pass-1.6.1-r1.ebuild, + pass-9999.ebuild: + Use proper bash completion eclass. +