Typically, a custom block without the minecraft:selection_box component will use the full block space for the selection. This component is not required. However, when a custom block has the minecraft:multi_block trait with no selection component, the selection outline will not be rendered when “hovering over” the block.

Demonstration

The attached screenshot shows the issue in question. No selection outline is rendered around the multi-block.

Reproduction

This issue occurs in any custom block that has the minecraft:multi_block trait with no minecraft:selection_box component.

The demo screenshot is 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.