So apparently entities become temporarily invulnerable when they take damage. It is not normally noticeable as it happens really quickly.

The easiest way to see the effect is by hitting an entitiy more than once at the same time.

For example by making multiple dispensers shoot arrows simultaneously or by shooting an entity from upclose with a crossbow with the multishot enchantment. The result is that only one arrow hits and the rest bounces off. (see images)

The same thing happens when shooting a lit TNT.

I assume that this is intentional to some extent and that there is a good reason for it but it would make more sense if all the arrows would hit the target.