The bug

Previously (as in 1.12), in the advancement icon tag, you could specify a "data": x and in your resource pack have that damaged item correspond to another texture in-game. With the flattening and the removal of the "data" in 1.13, this functionality has been compromised.

The changing of textures due to damage taken by a tool is an officially supported feature, and hence the removal of this feature may be an oversight by the developers, hence a bug.