Hi Marie and James,
Happy to see you move to version 17.0, you will surely be delighted to use it.
Coming to your question, the new layout designer introduced in the 64bit version has drastically changed over the years following the feedback from users. In the new versions the map window and the layout window are kind of independent of each other meaning when you create the layout from the map , you have the map cloned in the Layout window. A layout consists of frames that hold the content such as a maps, legends, scale bar, text, or table. The layout is designed by rearranging these frames.
If you want to have the Layout to be the same as your map you will have to make all the changes in the map window and then create a layout of it. Otherwise the changes made on the Layout will not reflect on the map window.
Hope this answers your question.
Regards,
Shweta