I used to be able to view what keystone my other characters had (same realm etc) when I opened my bags from another character, however now the slot where my M+ Stone is remains empty with a tooltip saying "Retrieving Item Information" when I hover over it. Works both ways when looking from my main to alt and vice versa, and persists when I have linked, moved or played around with the stones in any manner of ways.

+1

This is definitively an issue with the item server. Blizzard screwed up a lot of things with the item server...