luaposix isn't rebuilt when installing a new lua slot:
user $ emerge -qv -1 lua
[ebuild NS ] dev-lang/lua-5.3.3-r1 [5.2.3-r2] USE="deprecated emacs readline -static"
user $ cat /var/db/pkg/dev-lua/luaposix-33.4.0/RDEPEND
Since --complete-graph mode is the natural way to account for reverse dependencies, and the default --complete-graph-if-new-ver=y setting does not account for new slots, it would make sense to add a --complete-graph-if-new-slot option to pull in reverse dependencies for this case.