Zombies take two punches to kill when on half a heart

When a zombie is at half a heart (after falling 23 blocks), they require two punches to kill despite being at half a heart. Other mobs (e.g. Skeletons and Creepers) are killed with one punch when at half a heart.

What I expected to happen was...:
When I punched the zombie after it fell 23 blocks, I expected it to die with a single punch

What actually happened was...:
When I punched the zombie after it fell 23 blocks, nothing happened. There was no reduction in health, and I needed to punch it a second time in order to kill it.

Steps to Reproduce:
1. Make a zombie fall 23 blocks, reducing it to half a heart
2. Punch it once. It should not die and require a second punch

Notes:
A video has been attached to better demonstrate this issue.{}