The crack textures for iron golems are broken, every crackiness level looks identical to the high crackiness texture as if the "crackiness high" texture is overlapping the "low and medium crackiness". This can be reproduced by hitting a golem 4 times with a diamond sword which causes the crackiness high texture to be displayed instead of the low crackiness texture.
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Can confirm in 21w03a.
Can confirm in 21w07a. Feel free to use the following command to make reproducing this much easier:
/summon minecraft:iron_golem ~ ~ ~ {Health:74.0f,Glowing:1b}

Can still confirm in 1.16.5! The issue is still the hidden portions in the alpha channel.

Cannot reproduce in 21w16a. It seems to be fixed since 21w10a.
This issue was present in 21w08b, but no longer occurs in versions above or equal to 21w10a. With that being said, this issue has been fixed in 21w10a.

Can confirm fixed in 21w10a.
I figured out the reason this happens. The crackiness textures are correctly being switched between, rather than being overlaid on top of each other as this issue would imply. The problem is that the low/medium crackiness textures contain parts of the high crackiness texture hidden in the alpha channel. Their opacity is low enough for them to be nearly invisible in an image editor, but just high enough to appear at full opacity in-game, making the crackiness levels look identical. My fix for this was to just select all pixels outside of the intended visible cracks and set their alpha channel to 0. I attached a resource pack with this fix:
[media].
Fixing this also appears to fix MC-173834.