Yep, internal-face culling has several bugs, this being one of them:
-Adjacent wedge triangle-sides are not culled (as seen here)
-Internal full-block faces of heptas are not culled
-Borders between open door-type blocks and transparent non-door blocks DO get culled out, which can cause a lot of silliness.
-Annoyingly, borders between things like metal grilles/meshes and glass DO get culled out. This is, however, not a bug, and I have no idea how to solve it without breaking other things.