It changes the order of the Styles in Preferences/Layouts and Styles/Styles (which affects the order in Prefs STYLELIST) - moves it the bottom of the list for some reason.
There is a onerous fix for this - in that the Styles can be re-ordered in Preferences/Layouts and Styles/Styles
Pretty sure this is not by design or intended, but if so feature request to have an option not to change order of Styles
There are a few ways it can be done but, as far as I can tell, saving/updating an existing style will leave it where it is in the list without any problem.
OTOH, if you add a new style and then give it the same name as an existing style, then choose Replace when prompted, that gets treated like a delete-and-create, so the new style ends up at the bottom of the list.
(You can instead modify an existing style by right-clicking it in the Lister menu or similar, and choosing the Update option. Or a command like Prefs STYLESAVE="My Style Test" will keep the existing style's position as well.)