Spoilers below.
After the vampire fight in the Bone Keep there is an armor stand that can be looted. The loot on the stand is +1 half plate and +1 shield. They cannot be looted together. Looting them together with the same character will cause either the plate or shield to disappear. If you want both pieces of gear, you have to loot the stand twice. Loot it once for the plate with one character. Loot it again for the shield with a second character. I tried it a few times and the bug repeated an that was the only work around.
I could loot them both successfuly at the same time, but it was during combat. It's quite heavy ;-)
I second the OP; tried it three times. Whether identified or not, if I looted both with the same character, the first piece to be equipped would stay, the second would vanish. One would also vanish unless I equipped them straight from the container. I also had to resort to dropping my regular half-plate before equipping the +1, or my regular half-plate would vanish on swap out.
On top of all that, it appears each piece has +1 too much bonus. When I equipped them by replacing regular items of the exact kind, my total AC went up +2 instead of +1 for each item.
I was able to loot them both successfully at the same time after peaceful dialogue with Aksha.
I had a somewhat different looting problem in the keep, but it happened shortly after this fight, where I could not save, and every time I attempted to interact with anything the game would do nothing and not play the interaction animation. I had to tell the character to do something else, and eventually the PC would perform the animation before doing the new thing. It was very irritating.