I suspect that this is one of those rare occassions where a 'Do something' trigger is necessary to override the AI but hasn't actually been provided.

The AI seems to recognise that whilst it has no means to win the battle by attacking, it also has no reason to withdraw as it is actually more powerful than its opponent. Hence, it sits and waits in the hope that the situation will change and allow it to win.

What is needed is a 'Do something' trigger that identifies the situation and overides the AI insisting that it either makes a pointless attack or withdraws.

The only other fix I have had limited success with is to provide the AI with a fresh tactical situation so that it is persuaded to initiate an action itself. Its obviously risky but sometimes its possible to persuade the AI to attack by faking a rout within reaction range of its units, or to trigger the 'Attack missile troops' trigger and then kite the enemy into tower range.

Sometimes just outflanking or moving behind the AI line will be enough to trigger an AI re-assessment of the situation and prompt a withdrawal. I've certainly seen my son do that with his Hungarian Noble Cavalry.

Nevertheless, it is a bug and needs to be fixed.