Summary:
In Bedrock 1.16.0.51 and Java 20W06A there was a change in walls:
https://www.minecraft.net/article/minecraft-snapshot-20w06a
https://feedback.minecraft.net/hc/articles/360040975731

Walls do not have gaps anymore when stacked vertically

Now all the walls without a block on top form an even connection, that is, not in the form of a column. If you place a block on top, in Java the lower walls are instantly updated, turning into a column; in Bedrock this does not happen instantly, but sequentially, starting from the top block.

Steps to reproduce:

1. /fill ~ ~ ~ ~2 ~10 ~ cobbled_deepslate_wall

2. Place any wall block on the top middle wall block.

Observed results:
The walls are updated (turned into a column) one by one, starting from the top block.

Expected results:
The walls are updated instantly - all lower blocks.

Video:
walls do not update instantly.mp4