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 20
MC-123097
/setblock and /fill do not update block states
Resolved
MC-129184
Glass panes and Irob Bars don't update
Resolved
MC-129971
ironbars act funny (fill command)
Resolved
MC-130478
/fill doesn't connect glass panes to other blocks
Resolved
MC-131606
fiil and setblock command with iron bars, glass pane and stained glass pane does not behave properly
Resolved
relates to 3
Attachments
Comments 9
you could also add facing to it, for example /fill 2 4 2 6 6 6 orange_stained_glass_pane[north=true,south=true]
This seems to be a duplicate of MC-123097.
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.
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