Reproduced on MacOS in Unity 6.0.14 in both Editor and Play modes: a UXML template in a custom list view VisualElement disappears after the UXML template’s file was slightly edited. Note that the Unity standard ListView is not affected. Indeed, so long as a ListView using the UXML template in question exists in the same UXML document, all references to the edited UXML template will be correctly updated. This makes me think some cheap tricks are being pulled for the sake of ListView and ListView alone.