I put this in a second report. When using the live-mode, you dont want this message as its obvious that the document loaded. This also causes flickering, and does not really have any benefits. It can stay there in the regular, manually updated preview. But in automatic reload preview it should be removed.
The preview plugin is not maintained, if you want to have this improved, please submit a merge request, otherwise that will stay the way it is at the moment. https://kate-editor.org/join-us/ I am sorry that does create issues for you, but the only other fix would be to remove it.