Tested on TW:WH3 in custom battle, patch 2.0 (Update: still the same on 2.2).
A replay with examples is attached, important timestamps are listed at the end of the post.
Video illustrating the issue and comparing it with Warhammer 2: https://youtu.be/2berqXEhzw8
It seems like whenever an AI controlled unit comes into contact with an enemy unit that is considered higher threat than the currently engaged target, the AI almost instantly changes the attack target to the higher threat unit. For some reason a unit of the same type that the AI is already fighting, attacking on the right flank (and only the right flank as far as I can tell), seems to be considered higher threat as well. It is often enough that a few models touch the AI unit to trigger the target change.
The target change makes the AI unit focus 100% on the new attack target and the individual soldiers will not fight back against the previous target unit, or any other unit for about 25 seconds. After that much time has passed the unit seemingly "unlocks" and the soldiers are allowed to fight back against other units than the targeted unit.
It looks like the same ~25 seconds of single unit 100% focus is true for player controlled units that just received a new target as well so that part is probably not AI related, but in combination with the AI's targeting behavior it causes some pretty serious issues in my opinion.
An example of this is that when an enemy unit is charged from behind with a high threat cavalry unit, the AI unit will switch attack target to the cavalry unit and then begin chase after it when it is withdrawn. This causes the AI battle line to pretty much fall apart instantly.
Replay timestamps that demonstrate these issues:
01:30: Several of the dwarf units just barely touch a grave guard unit on its right flank and instantly switch target, causing the soldiers to ignore the unit right in front of them until about 01:50 when they start fighting back again.
02:25: Two dwarf units leave the line and chase after the Blood knights after a very short charge cycle
03:30: Another two dwarf units chase after the Blood knights