For Reference Document naming series & Document naming rules is responsible to design or generate user specific code check twos DocType.
Your Problem @matthkarl
2. Made two rules: is_sales_item = Yes + item_group = Events
Instead of
is_sales_item = 1
item_group = Events
Make sure item group name same in item group list.
System will check case-sensitive manner wile triggering this.
Hope you understand…!