The bug fix to Armour Explosion didn't just stop chain reactions, it was a MUCH bigger nerf. Here is why:

TL;DR: Because of how PoE 2 seems to process hit ordering, the Devastate/Armor Explosion combo will no longer trigger on every enemy you Heavy Stun in your active skill's AoE.

First, I'll explain how/why Armor Explosion was changed. From Patch Notes:

Fixed a bug where the Explosion from the Armour Explosion Support could trigger itself recursively.

Armor Explosion specifies that the triggered explosion "Cannot break armour". However, there was (at least one) workaround to this by using the Devastate support gem. Devestate causes linked skills to fully break the armor if that skill inflicts a Heavy Stun, and linking Devastate and Armour Explosion to the same skill allowed for them to support each other and (for some reason) bypass the "Cannot break armour" effect to allow for the triggered explosions to chain react as long as they were Heavy Stunning (this was easy to do with some investment).

This was definitely a bug because it was inconsistent will other sources of "Cannot break armour," namely Leap Slam and the Exploit Weakness support gem. Linking Devastate to either of these gems and inflicting a Heavy Stun does not break armor, meaning that Armour Explosion was an inconsistency.

However, what I don't think was intentional was just HOW MUCH WORSE ARMOUR EXPLOSION GOT AS A RESULT OF THIS CHANGE!

I'll explain why by showing a few GIFs of me using Rolling Slam linked with Armour Explosion and Devastate in the latest patch:

https://i.redd.it/tm46keczaj7e1.gif

In this first example, Rolling Slam is hitting (and Heavy Stunning) the 2 skeletons in the middle, with them far enough away from each other that their Armour Explosions don't hit each other. We see 1 explosion from each of them for 2 explosions total. If this was pre-patch, we would see 4 explosions, as the other 2 skeletons that weren't hit by Rolling Slam would still have been Heavy Stunned by Armor Explosion, triggering a Devastate Armour Break into another Armor Explosion trigger for each of them. Only 2 explosions is how the Devastate/Armour Explosion was always supposed to work (based on the description), with every enemy you Heavy Stun with the active skill having their armor broken and triggering an explosion, but enemies hit by the just the explosion will not have their armor broken.

So if we want to trigger an Armor Explosion, we must actually hit the enemy with the original skill. Let's try this again, except we hit 3 enemies bunched together with Rolling Slam:

https://i.redd.it/0yav9k39bj7e1.gif

Here, Rolling Slam looks like it is hitting (and Heavy Stunning) all 3 skeletons, BUT WE ONLY SEE ONE ARMOUR EXPLOSION TRIGGER. Though this GIF is just one example, I've tested this interaction a bunch more against much tankier enemies and can confirm that if the enemies are too close together, then Armor Explosion won't trigger on every enemy Heavy Stunned in Rolling Slam's AoE.

Why is this? Obviously I don't have the source code, but from reverse engineering I think I have a pretty good idea why. I believe it has to do with how PoE 2 seems to process the order of hits.

In this case, when the AoE hit from Rolling Slam comes out, it seems to first check for the "closest" enemy to the center of the AoE. This enemy gets hit, and the damage/effects from this singular hit gets processed BEFORE any other enemy in the area gets hit by Rolling Slam. To put it another way, when you use any AoE skill, the first enemy processes its damage and effects (including the Heavy Stun -> Armour Break -> Armour Explosion combo) BEFORE any of the other enemies in the original AoE skill take damage from the original AoE skill. This means even though ALL of the enemies are in Rolling Slam's AoE, most of the enemies are going to be hit a triggered Armor Explosion BEFORE they get hit by Rolling Slam.

In the above GIF, we aren't seeing 3 enemies getting hit by Rolling Slam, we are seeing 1 enemy get hit by Rolling Slam and the other 2 enemies getting hit (and killed) by the first enemy's Armour Explosion. THIS IS VERY BAD FOR US because any enemies Heavy Stunned by Armour Explosion now have the "Immobilize Resistance" mechanic, meaning we probably aren't stunning them with the hit that can trigger the Devastate -> Armour Explosion combo. This wasn't a concern pre-patch because either hit could trigger Devastate, but now we would need to hit an impossibly difficult sweet spot in order to trigger Armor Explosion on clustered enemies.

But this is totally fine because it's not like the whole purpose of Armor Explosion is to deal with clustered enemies, right? RIGHT?

Admittedly, the Devastate/Armour Explosion combo isn't entirely worthless, as more stun resistant groups are more likely to trigger more explosions (e.g. Magic packs), meaning we get the damage where it counts the most. However, because the enemies farthest from the center of your AoE attack should be the lowest priority for hit ordering, they are almost certainly going to be Heavy Stunned by another enemy's explosion before you get a chance to Devastate them, reducing our overall AoE clear potential much more than just stopping the chain reactions.

Overall, the new patch made Armor Explosion really messy and inconsistent and I really hope GGG addresses their (likely unintentionally) heavy handed nerf to an archetype that didn't deserve to be made even slower.