Add an item to a list for Rules and References
The "add an action" form says: "To make entity fields appear in the data selector, you may have to use the condition 'entity has field' (or 'content is of type')."

My field was in the data selector but the item was not added by the action.

I changed the action to a rule with a condition "entity has field" as requested, the field being the one I wanted to add a value to. Then I recreated the action in the actions area.

This was the solution for me. Hope this helps.