I am using Microsoft 360. When I have a workbook open, then try to open another workbook, excel places the newer workbook, not on top of the older one, but several lines down on the screen. Therefore, I always have tabs at the bottom or the workbook that I cannot read as they are off of my screen. The remedy is a nuisance, having to resize the newer workbook to make it fill the screen and be centered top to bottom. Why is the new excel doing this?
I'm not the Excel user-interface designer, just another user like you. I kind of like the feature you're complaining about, because when I open several sheets at a time, I can see that they're all open; that would not be the case if they all just stacked up. That said, I can see the inconvenience you describe as well. You can always give Microsoft the feedback....and maybe there will be somebody else here who can show you how to change some default setting. In the meantime, don't let it upset you too much.