Custom blocks support different rendering depending on context using the minecraft:item_visual component. Some undetermined issue is causing rendering to be incorrect in item contexts using this component. It appears that data from the minecraft:material_instances component may sometimes be taking priority over the item visual component.

Demonstration

The following block’s definition is supposed to be using opaque in-world rendering and blended item rendering:

Notice that the held item render method is opaque instead of blended.

The reverse situation, where a block is supposed to use blended in-world rendering and opaque item rendering, is also incorrect:

In this case, both the held item and icon are incorrect.

Reproduction

The first example above uses the following block definition:

{
	"format_version": "1.26",
	"minecraft:block": {
		"description": {"identifier": "bug_rendering:custom_block_with_differing_material_1"},
		
		"components": {
			"minecraft:geometry": "minecraft:geometry.full_block",
			"minecraft:material_instances": {
				"*": {"render_method": "opaque", "texture": "glass"}
			},
			"minecraft:item_visual": {
				"geometry": "minecraft:geometry.full_block",
				"material_instances": {
					"*": {"render_method": "blend", "texture": "glass"}
				}
			}
		}
	}
}

The second simply reverses the render methods.

These examples are taken from the behavior pack in the following world attached for convenience:

This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such. Also included are demonstrations of proper behaviors via the following 2 definitions:

• bug_rendering:basic_custom_block

• bug_rendering:custom_block_with_same_material