It is a bug.

Current behavior: directive ngValue sometimes do not update value attribute. In my examples below, I clear $rootScope.someText value with delete operator (or set $rootScope.someText to undefined) and ngValue directive do not update view value.

Demo:
Example in CodePen: https://codepen.io/dmitry_krekota/pen/NdNYjr
Screen record with bug reproduce steps (12 seconds): https://drive.google.com/file/d/0Bxtv10NlSOEbeXJWWnhZc1A0anM/view
Also, you can download the html file with demo: https://cl.ly/1h0F3l2P032K/download/demo.html

Expected behavior: ngValue must set attribute value to '', when when there is no value in $rootScope.someText variable.

Environment: Windows 7, Chrome 56

Angular version: 1.6.1