In versions prior to the singleplayer/multiplayer merge (before 1.3), the heart bar would briefly flash faint hearts depending on how many of them were lost when taking damage. This is gone since version 1.3, with no mention in the change logs. Here's a video comparison of the health bar between 1.2.5 and 1.4.2.

What I expected to happen was...
Blinking faint bright hearts showing how much health I had before taking damage. If I had 10 hearts, and I took 2 hearts of damage, I'd expect those 2 hearts to blink briefly before dissapearing completely.

What actually happened was...
No sign of how much hearts were lost. The hearts completely dissappear just after taking any damage.

Note:
This is not about regeneration, which works as expected. User Vincent Lee has found a relevant part of the code related to the bug.

Steps to reproduce:
1. Load a world on survival mode.
2. Take damage from any source.
3. Observe the heart bar. It should flash the hearts that were lost, but it does not.