Which version of Lightroom are you using? Lightroom Classic or the cloud-based Lightroom? You've posted your question in the forum for cloud-based Lightroom, but only Lightroom Classic has an option to automatically add the exported jpegs back to the catalog, so I'm assuming it's Classic that you are using. In which case, look at the export settings and in the very top "Export Location" box there's a checkbox called "Add to This Catalog", obviously this needs to be unchecked if you don't want the exported file to be added back to the catalog.
If that's not the issue, let us know and start with the Lightroom version number that you are using.