Adding a non-abstract embedded element from UI creates a new element every time form is rebuilt

Non-abstract plugins created using the default "add" action are created as many times as the form is rebuilt. The consequence is that plugins with parameters not restricted to an input mode are recreated every time the input mode is switched. So, if the "add" form starts out with a parameter in input mode, the user switches to selector, picks a variables, and then saves, two of the elements have been created, with the first potentially failing integrity tests (since no settings have been provided).

To demonstrate this problem, create a "switch" (Conditional Rules) and go to the form for adding a case. Switch the input mode back and forth several times before picking a value and saving. There will now be as many new cases as number of times switched. Every case except the last has an integrity error.