The issue was item wasn't set to stackable at creation and yet when opening a loot bag it was set to give 10 of them. And it didn't warn that you cannot have more than 1 because item wasn't set to be unique (1) so at creating loot bag system didn't warn and at event time it would simply fail. I cannot be sure 100% that it is the coz, but Item having stack of 10 been created was the issue for sure.

Checks should be put in place to prevent non-stackable items from being put as stacks in the loot effect, as well as analysing if other checks aren't verified during edition.