This is the 3rd time this has happened recently, but one of the times the stack of items reappeared an hour later. No, my account wasn’t hacked. A sizeable amount of gold was invested into the stack that the game ate or ‘disappeared’ and not something easily replaced.
PS: Stackable items do not seem to be recoverable by item restoration.
Edit: Lovely, a second, but much smaller stack of chrono bolts has now also disappeared.
2nd Edit (over 1 hour later): What Wow takith away, wow givith back. Both stacks are now back in my inventory.
I’ll note that this has now happened four times that I’d noticed and at least in one instance did not return… Thus far I’d only noticed things disappearing in my reagent bag slot .