When I use format: "es" & sourcemap: false, bundle.write outputs an es module and a systemjs module with a sourcemap

[twilson90]

Can't see anything in the documentation about this behaviour, or anyone else asking the question.
Why is this happening?

let output = {
        "format": "es",
        "dir": "dist/assets",
        "sourcemap": false,
        "entryFileNames": (info)=>{
          return `${get_name(info.facadeModuleId)}.${ext}`;
        },
        "chunkFileNames": (info)=>{
          var id = info.facadeModuleId || info.moduleIds[info.moduleIds.length-1];
          return id ? `${get_name(id)}.${ext}` : `[name].${ext}`;
        }
      };
      await bundle.write(output);

[lukastaegert]

I would suggest checking all plugins that you use.