Guild bank missing items - what happened?

[Beefums-elune]

Blizzard finally made a comment on this.

Guild Bank Missing Items Update

About a week after we launched patch 11.0.2 in August, we began receiving reports of an issue where some players were discovering items missing from their guild banks. As we investigated over the next couple of days, we found the culprit—a technical update that was made to support cross-realm guilds. An unexpected bug caused one of our maintenance processes to make some items disappear. Many of these items were related to professions materials from prior expansions, but it was possible for play…