Mixer 2020 now only supports the Metalness workflow. Old Mixes made on the Specular workflow will automatically be converted to metalness, unless they contain paint layers. In case of paint layers, diffuse and specular will not be loaded, instead empty albedo and metalness maps will be created.
You can find this information, and more, in the Quixel Mixer User Manual. You may also access it by opening Mixer > Help > Manual.