Assuming the default setup in LibreOffice, when users save some document to a non-ODF format they are warned about potential data loss. Nonetheless, they can change the default format at Tools > Options > Load/Save (General) and no warning is issued at that point, nor will be when they save their documents. I think we should also warn users about potential data loss in the moment they change the default format. Related: #90478.
Ok, let's set to NEW.
I consider to be normal, and not enhancement base of the fact that there could be data loss.