Ugh. You Mojang guys are literally just replacing one bug with another. ;-;
What I expected to happen
Glass panes would trigger the cullface of the tops and bottoms of other glass panes of the same type, like they did on previous versions before the 1.13 snapshots, and also like they do on all other faces of the glass panes.
What actually happened
Glass panes don't trigger the cullface of the tops and bottoms of other glass panes of the same type, like they did on previous versions before the 1.13 snapshots, and also like they do on all other faces of the glass panes.
How to reproduce
Download and enable the attached resource pack that adds cullface to glass panes
Place a glass pane on top of another glass pane of the same type
Notice how Mojang refuses to completely fix this bug
Related issues
Attachments
Comments

Could you attach a small sample pack so this becomes easy to test?

Done.

Now the bug is finally completely fixed.

Affects 18w47b, and if I remember correctly, the 1.13.2 snapshots. Attached pack acts weirdly, though, as some states use cullface when they should not.
I would also consider this blocked by MC-109087.