The bug
The /fill
and /setblock
commands are broken when used with iron bars, glass panes, stairs and fences. As you can see, iron bars don't attach to nearby blocks, this is the same with fences.
Linked issues
is duplicated by
relates to
Attachments
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you could also add facing to it, for example /fill 2 4 2 6 6 6 orange_stained_glass_pane[north=true,south=true]
Confirmed for 1.13.1.
This seems to be a duplicate of MC-123097.

Occurs in 1.13.2 too!
[media]They also don't trigger observers when a tree grows next to them despite visually changing. This used to work (via bud mechanics) until extended block states were introduced, but since the flattening removed fences extended state, this should once again work.
I'm not sure if this deserves it's own post or just as a note on this one, they seem very similar.

CAnnot reproduce in 20w30a
I am no longer able to reproduce this issue in 20w51a.
I think this was fixed in 1.16 pre-release 6, same version when MC-191096 and other similar issues became a thing.
Most probably same cause as MC-123087