In macOS Mojave only the filename was selected in the Save / Save As... window.

Now nothing is selected so you can't start typing right away or the extension is also included in the selection. So if you need to save multiple versions or need to add a number at the end of a filename you could hit the Right Arrow key and start typing.

If you do that now the cursor moves after the extension, you have to move 4 positions back or delete the extension entirely and then start typing.