Avoid making deep copies of list items as well as deep watching collections, which also leads to copying. Instead, maintain a shallow copy of the source list so that any shared references nested in the list are undamaged by copying.
Delay 100ms after blurring a field before testing it against the placeholder and removing it. There were issues with mouse clicks on the typeahead suggestions firing after the call to updateItems which caused the value to be removed rather than added.