According to the documentation QHelpEngineCore::findFile should return an invalid url if the resource reprented by the url cannot be found. Currently the url that is passed to the function is returned unaltered in this case. So passing a valid url to a non existing resource to the function will result in returning a valid url. This was working as expected in 5.11 and seems to be broken since https://codereview.qt-project.org/#/c/203587/