The minecraft:geometry.full_block hardcoded model was updated in Preview 26.0.27. This update seemingly fixed all issues but one: rotation of the cube in item frames and shelves.

Workaround possible

This bug can be worked around using the minecraft:item_visual component.

Demonstration

The following screenshot shows 2 blocks, a Vanilla oak planks block on the left and a custom lookalike on the right:

The lookalike is leveraging minecraft:geometry.full_block. Note that its top face is rotated 180°. Unseen is that the same is true for the bottom face. Presumably, the full-block hardcoded geometry needs to be rotated 180° about the y-axis for this item display transform.

Reproduction

The following trivial block definition showcases this bug:

{
	"format_version": "1.26.0",
	"minecraft:block": {
		"description": {"identifier": "bug_cube:custom_block"},
		
		"components": {
			"minecraft:geometry": "minecraft:geometry.full_block",
			"minecraft:material_instances": {"*": {"texture": "planks"}}
		}
	}
}

This example is 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.