(This is new to the 1.16 snapshots added with the vertically connected walls.)

When placing walls down multiple blocks high, if you replace one wall block, all blocks below it get turned into endcap blocks. This only happens on walls aligned in the East/West direction. North/South aligned walls work as expected, and the lower wall blocks remain as middle wall blocks.

Repro Steps

• Build a wall along the East/West axis that is at least 3 blocks long and 2 blocks high
• Break any of the upper blocks of the wall that are not the two end cap blocks
• Replace that block with a wall of any type (does not matter what the original or replacement wall types are)
• Note that the block below it turns into an endcap block instead of remaining a smooth wall
• If you break and replace the wall block below the one that was replaced, it returns to being the correct middle wall segment