The bug

Walls will connect vertically (i.e. raise) to several blocks which they (alongside fences, glass panes and iron bars) are coded to not connect to horizontally.

These blocks comprise the barrier, all 17 shulker boxes, all 8 leaves, pumpkins alongisde both their variants, and melons

How to reproduce

1. Place a fence, wall, iron bars or glass pane next to either a shulker box, leaves, barrier, pumpkin or derivative, or melon
   1. Note that unlike with most solid blocks, they refuse to make a connection
2. Now place one of those blocks on top of a wall which has a horizontal connection to somewhere else
   1. Note that the wall rises to meet this block, despite it not making connections to it horizontally

Expected result

The wall would not raise and meet these blocks, since they're not considered valid connections horizontally.

Actual result

The wall seems to connect to it anyway.

How to fix

I'd recommend making the list of solid blocks that fences, walls and other such blocks refuse to connect to a tag, as its current hardcoded implementation is quite messy. Even if this behaviour is considered to be working as intended, I still strongly recommend making this into a tag just for horizontal cases.

The proposed block tag, #minecraft:disallow_connections, would list all solid blocks that fences, walls, glass panes and iron bars will not connect to horizontally, and, if this ticket is considered to be a bug, that walls will also not connect to vertically.

It contains the following blocks and block tags:

• minecraft:barrier
• #minecraft:shulker_boxes
• #minecraft:leaves
• #minecraft:gourds (a second new tag that contains minecraft:pumpkin, minecraft:carved_pumpkin, minecraft:jack_o_lantern and minecraft:melon)