CK II - Attached armies don't participate in the battle of the top-level army 2.8.2.1

[hire-mew]

One line summary of your issue
Attached armies don't participate in the battle of the top-level army 2.8.2.1

Game Version
2.8.2.1 SAHQ

What expansions do you have installed?

Monks & Mystics,
The Reapers Due,
Conclave,
Horse Lords,
Way of Life,
Charlemagne,
Sons of Abraham,
The Old Gods,
The Republic,
Legacy of Rome,
Sword of Islam

Do you have mods enabled?
No

Please explain your issue is in as much detail as possible.
Top-level army (T shaped chain icon) has several armies attached (horizontal chain icon)
Top-level army is AI controlled.
Top-level army is marching into a province with units hostile to it.
The attached armies are neutral to these units, only the top-level army sees them as hostile.
A battle commences. Only the top-level army is the participant of that battle and the attached armies stand in the background (still with the chain icon) and keep following after the battle.

This isn't how it normally works, but it happens sometimes.
You know it's a bug when the AI also expects the attached armies to be fighting and they don't.
Because an AI army of 150 soldiers never attacks an enemy stack of 3000 soldiers. Unless it has a pathing problem and is itself attached to a distant army. It does so when it has 3000 allied soldiers backing it up.
The top-level AI army decides if it's strong enough to attack the enemy based on the number of troops in the entire chain with it.
But sometimes the AI gets duped by the game rules concerning who's hostile and who's neutral to who.
See my screenshot.

In this scenario the same thing happens when the top-level army tries to besiege a province occupied by enemies which are neutral to the armies in the chained stack. They expect to have enough troops to besiege it, but the attached armies don't count toward the number of attackers. So the AI just sits there with their mini army doing no siege progress. Being as surprised as human players that hostility rules of the top of the chain stack are not respected.

In practical terms there is no way around this problem. Because it was your game design choice that the Offer to join war option is not there, when you're already fighting with a person on the same side in some war.
To mitigate that, you're always able to attach to their army and help them in their other wars. But if you can't fully rely on this mechanic, then it's a big mistake.

The key is in the Steps to reproduce the issue.
It explains the condition under which the mechanic breaks.

Explanation of screenshots:
attach1.png - the battle - only Leon and Asturias are fighting despite the rest arriving there attached
attach2.png - the war in which linked armies can help normally (participating or not)
attach3.png - the war which linked armies can't help him with (unless participating)
attach4.png - Umayyad revolt to show that attackers of both wars although initially neutral became hostile to each other.

Steps to reproduce the issue.
Not easily reproducible for me. The issue persists even beyond save/load. We know quitting and starting the game again fixes many AI issues, but this is not the case.
Try taking a unified muslim Spain. Two emirs (B&C) ally up and attack Asturias. Then single emir (D) attacks Asturias. Ie. two wars for the same title. You as a christian ruler join the war on side of Asturias against D (the single emir). Then Umayyad revolt must happen, where the emirs B & C stay with the sultan, and the emir D is the leader of the revolt faction. Ie. the two attackers are now hostile to each other. Relations are like this:

Yourself / Asturias = allied
Asturias / emir B = hostile
Asturias / emir C = hostile
Asturias / emir D = hostile
Yourself / emir B = neutral
Yourself / emir C = neutral
Yourself / emir D = hostile
emir D / emir B = hostile
emir D / emir C = hostile
emir B / emir C = allied

Now you have your army attached to Asturias. The B&C have their armies attached together.
When Asturias marches into B&C unit and a battle starts, your army will stay attached but will not participate in the battle.
When Asturias tries to siege a province occupied by B&C, your army will not count toward number of attackers in the siege screen. As if you were really neutral and not attached.

You might argue that the enemy of my enemy is my friend. But they are not a friend to the army I'm attached to.

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[hire-mew]

Another example. I'm Foggia
Arrived there attached the same day as the rest, and nonparticipants aren't fighting. Pretty much a war losing event.
Even though few months earlier we fought this very enemy, with this exact stack without any problem. Nobody detached/reattached. Only thing that happened is that AI reinforcements arrived, merged, etc. Same for the enemy.
You have my savegame in the cloud. You see the date. You can have a look.

 

[hire-mew]

Next battle again Asturias and Navarra fought alone, while their allies were sitting it out.
When the retreat starts the allied armies follow them since they are still attached.
Funny thing happens when their shattered retreat takes them through a province with enemy units in it.
Since the allied units were not defeated, they don't have the shattered retreat flag. Only the top of the chain does, so they will pass the province without harm, while the allied units will die.

I detached.
- the attach button isn't greyed out, but clicking it doesn't do anything (not attaching me back)
- the button tooltip is wrong (it says Navarrese army, which isn't the unit at the top of the stack) - exhibit 1 and 2
- the armies shattered retreat into a battle even though that province isn't the destination of the shattered retreat - exhibit 3 and 4

Are you starting to get a feeling this is a nasty problem?