When a player tries to break the dragon head, no cracking animation shows up. However if you position yourself right on the corner so you can see inside the head, the cracks do appear on the small head hitbox inside the dragon head instead of on the head itself. This explains why the Dragon Head doesn't show cracks on the outside.
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This issue relates to MC-82961.
There's a smaller hitbox in the size of a skull inside of the rendered dragon head which displays the cracking texture.
Can confirm for MC 1.12.1.
Confirmed for 1.10.2