Description:
In all three dimensions, skeletons will appear to have a pure white skin after not restarting or quitting game for some time(about 2 days, most of the time game is paused or at title screen). This applies to the skeleton head as well. The stray variants are not affected.
Solution:
Can be temporarily fixed by restarting game.
Related issues
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Comments

[media][media]
This appears to me a duplicate of MC-188522
Thanks! That seems to be the case. But I wonder if there should be a ticket for mob heads with no textures.