When the builder saves a project brief, it can fill a blank app description from the brief's one-line description of what the app does. This does not make a separate AI request just to write the description.
An existing description you entered is preserved by that fill step. Editing the description in Settings is the direct way to choose the wording you want.
If the description is blank, check the project brief. Where Overview offers Use project brief, that action can fill it from an existing suitable brief. A missing or placeholder brief line cannot supply a useful description.
A remixed app starts with inherited information. When the builder rewrites an inherited brief for the new app, it can also refresh that inherited description. Check the result after customizing a copy.
The app description is not every page's SEO description or social title. Check your page metadata separately before sharing the app.