Do you have the latest Office 2011 update, which updates all Office apps to version 14.5.5?
I have Word 2011 running, with 3 documents open. I running El Capitan GM.
When I cllck on the Word icon in the dock, I see all three documents listed, as usual.
I'm not sure what you may have changed, but I don't think it's a result of an OS X update.
Maybe something else that you have installed that has changed how the dock works... There's a few apps/utilities that can change how the dock appears to work.
That being said - you could also try downloading and installing the Yosemite combined updater. That is a tip that has worked for years to fix minor issues that appear in OS X, particularly after allowing the software update to install automatically.
You can get the
combined updater for Yosemite here