The Bug

An issue related to custom item models when the model scale of the displayed item is flipped to match a value strictly less than zero, resulting in that item to render shadows and light inconsistently. In addition, certain faces of the affected model also become transparent due to the back-face culling inversion.

This issue was still present and occurred in 1.15.2 as a weird lighting bug (similar to MC-163242).
 

How to reproduce

Demonstrations are within the resource packs provided :

• Fishing Poles Example (updated): The issue is noticeable when equipping a warped fungus on a stick to the right hand in first or third person.

  First person

        "firstperson_righthand": {
            "rotation": [-5, -85, 10],
            "translation": [1.75, 4.75, 2.25],
            "scale": [1, 1, -1]
        },

  Third person

          "thirdperson_righthand": {
            "rotation": [0, -90, 0],
            "translation": [-1, 3.25, 1.75],
            "scale": [1, 1, -1]
        },

Source : warped_fungus_on_a_stick.json

• Item inversion test (1.20-pre1) : The issue is noticeable when displaying a block item on the ground, in an item frame or a GUI slot.

  Item frame

        "fixed": {
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 0, 0],
            "scale":[ 0.5, -0.5, 0.5 ]
        },

GUI slot

        "gui": {
            "rotation": [ 30, 225, 0 ],
            "translation": [ 0, 0, 0],
            "scale":[ 0.625, -0.625, 0.625 ]
        },

Ground / Thrown item

        "ground": {
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 3, 0],
            "scale":[ 0.25, -0.25, 0.25 ]
        },

Source : block.json