Thank you Moonshine for alerting me to this post. I noticed that change when I upgraded from 2003 to 2016 a few years back.
The challenge I now have is parallel instances of Word running for each document I open, not merely separate windows of the same instance. Whenever I open a document (other than through Alt-F O) the blue "Word Office 365" splash screen appears followed by the new instance of Word. When the document is closed (Ctrl-W or Alt-F C), the empty instance remains: Word does not switch to the next open document. This bug only occurs with Word, not with other MS Office programs that I use.