Certain Vanilla blocks do not respect the minecraft:support shape configuration in a block definition. The full list not respecting the shape is unknown, but it at least includes ladders.

Demonstration

The attached screenshots (attempt to) demonstrate the issue. The custom block is configured to use the same shape configuration as a Vanilla fence, which only supports specific types of blocks on its up and down faces and no attached blocks on its side faces. For example, torches (correctly) cannot be placed on the sides of the custom block. However, ladders can be placed along the custom block’s sides. This is incorrect, as ladders cannot be placed on the sides of a fence post.

Reproduction

This demo screenshots are taken from the behavior pack in the attached world for convenience. This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such.