The bug

In 18w50a, most projectiles that have a "2D" texture now have an Item tag, which chooses the specific item stack to render. However, this change was not applied to the ender dragon's fireballs. While one could argue that this is because Mojang did not change mob-exclusive fireballs, big fireballs fired from ghasts do have the Item tag.

What I expected to happen:

The Item tag would work on dragon fireballs

What actually happened:

The Item tag didn't affect the dragon fireball at all

How to reproduce:

1. Summon a fireball with Item set to "{id:ender_pearl,Count:1b}"
2. Summon a dragon fireball with Item set to "{id:ender_pearl,Count:1b}"