As you probably know the best AC that a creature can get is -20 (which doesn't include bonuses from dexterity and single weapon style). Any bonus that wants to lower that value is being neglected by the engine (tested).
Now lets see what items IA offers:
1)Huskar Lord's Armor +14 AC bonus (or simply -4, doesn't matter is this case)
2)Supreme Shelter +7
3)Improved Ring of Gaxx +5
4)Talisman of greater protection +2
These already hit -20 minimum and any other AC bonus will be nullified. It's a rough example but it shows the problem well.

However AC bonuses vs single damage type (slashing, piercing, etc) aren't being counted there (tested).

So, unless it was intentional in the first place, I recommend to use four different effects (one for each AC type) instead of one (which gives a bonus against all weapon types).

Using different effects for different weapon types, will work but the true AC won't be shown in the character's sheet. The character's sheet will still show the AC which the character had before wearing the item. (So, it's not a good suggestion.)

Example: Consider a naked character with 10 AC. If you give him a +1 shield, the shield will improve his Ac by 2 points and you will read in the character's sheet that his AC is 8. Now if we change the shield and divide its AC bonus effect into separate AC bonuses for each weapon type (and cover all weapon types), the shield will do the same thing in practice, but when the character wields the shield, you will read in his sheet that the AC is still 10.

Moreover, I don't see any problem in the first place. The AC is capped at -24 (for those who wield shields) and -26 for others. Even if a character has too many protection items, he won't receive a better AC. That's normal.