When you hover the AC value in the inventory, then change your armor, then hover again, you can see that the breakdown list has modifiers from both the old armor and the new armor. To reset the list, you have to close the inventory and reopen it.
P.S.: Half Plate of Sturdiness (+1) seems to add its +1 modifier again, giving it the same AC as Half Plate of Robustness (+2). The +1 modifier for that armor appears in the breakdown, even though it's already pre-added.