This wasn't intended, the AI should immediately activate attack states as soon as it senses damage from a player. This bug was fixed in version 1.1.0 Update 1.
The error is related to the pistol creating a "Line Trace" with the "Visibility" type instead of "Camera." You can change this by editing L1_Player (called BP_Player in newer versions) and going to Macros under "InteractionSystem." In the "LineTraceByChannel" node, select "Camera" instead of "Visibility" in the Trace Channel section. Then, the AI and other objects that sense damage will react to the shot.