This page: https://qt-project.org/doc/qt-5/qtquick-modelviewsdata-modelview.html
Last section Using Transitions has following statement:
Additionally, changing the opacity of items to zero will cause them to disappear using the remove transition, and making the opacity non-zero will cause them to appear using the add transition.
This is incorrect – changing opacity leaves element in the Positioner. However changing it's visible property acts as removal/insertion.