Something is not right with {PRODUCT TAG}, and there is no fix for that.
But, we can do a work around. Basically, when a weighed item is added to a ticket, we add the unit to {ITEM TAG}.
Place {ITEM TAG} in the printer template and it will print the unit.
2 Actions are needed
Name : Add Item Tag Kg
Action : Update Ticket Item Tag
Tag : Kg
Name : Add Item Tag Grams
Action : Update Ticket Item Tag
Tag : Grams
Create a Rule
Name : Add Kg To Item Tag
Event : Line Added To Ticket
MenuItemGroupCode : Cakes ..or.. MenuItenName : Mud Cake
Action : Add Item Tag Kg
Having the items grouped is the best way otherwise you need a Rule for each Item.