Neatlines in an ArcMap map document (.mxd) are converted to rectangle graphics during the import process. Rectangle formatting is different from neatline formatting so the converted elements may not look exactly the same. In ArcMap, neatlines are often used to divide a layout. In ArcGIS Pro, rectangle graphics are used to divide a layout. Neatlines exist in grids to define the geographic data extent of the map.
Select the rectangle element or elements named Neatline and adjust the symbology as needed.