If a plugin uses one of them (the original) then Ardour will NOT delete the UI instance when it is closed.

If a plugin uses the other (the version "forked"/"copied" by falktx) then Ardour WILL delete the UI instance when it is closed.

Both specifications state that the UI is defunct and no longer usable after being closed:

   * After this callback is called, UI is defunct. Host must call
   * LV2UI_Descriptor::cleanup(). If host wants to make the UI visible
   * again UI must be reinstantiated.