[1.21.3 Bug] Mob head CEM does not apply in first-person or the inventory

[SeaOfPixels]

Description of Issue

There is a long-standing bug where mob head CEM does not apply in the inventory or in first person via handheld, but does apply when placed and when worn. This has been a bug for a long time but I guess it was never reported, affects 1.21.1 as well as versions before then.

OptiFine Version

OptiFine 1.21.1 J1, OptiFine 1.21.3 J2 pre5, OptiFine 1.21.3_HD_U_J2_pre6, OptiFine 1.21.3_HD_U_J2_pre9

Screenshots

Creeper head CEM example

{
    "textureSize": [64, 64],
    "models": [
        {
            "part": "head",
            "id": "head",
            "invertAxis": "xy",
            "translate": [0, 0, 0],
            "boxes": [
                {"coordinates": [-4, 0, -4, 8, 8, 8], "textureOffset": [0, 0]},
                {"coordinates": [0, 0, -4, 0, 8, 8], "textureOffset": [24, -8]},
                {"coordinates": [-4, 0.01, -3, 4, 7, 6], "textureOffset": [42, 19]},
                {"coordinates": [-4, 0, -4, 8, 8, 8], "textureOffset": [0, 32], "sizeAdd": 0.5}
            ]
        }
    ]
}

[SeaOfPixels]

May relate to https://github.com/sp614x/optifine/issues/7754.