I was in a party with a friend, and he noticed that he did not have many runes of shattering, and that his count went down after a bit even though he used none. I used one of mine, and his number of runes of shattering went from 4 to 2. (Mine went from 9 to 8).
Can confirm - I was the friend here. Been wondering why for several nights, it seemed like I never was increasing my Runes of Shattering when I knew I was picking up a bunch…we tested as reported above and my count when down. It really should be something like 30 total at this point, if not higher.
Can also confirm that I experienced this multiple times today between 05:30-07:30 UTC, entire play session was with only one other person and I consistently was using up all of their shatters.