The bug
The block breaking crack texture/animation/overlay on zombie heads and players heads looks different from how it appears on other heads.
It appears with two layers, even on parts of the head that don’t have an opaque top layer.
How to reproduce
Place a skeleton skull and zombie head
Start mining both of them
Expected result
It should look the same
Observed result
The zombie one looks different
Notes
This issue appeared some time in 1.13. 1.12 does not have this issue.
Can confirm for 26.2.