My coworker and I have identical Mac Pros with OS X Sierra v10.12.6 and Office 2016 with the newest build and Suitcase Fusion v19.0.2.
She has this issue with every Word, Excel or PowerPoint file she tries to open. I don't have any issues. On her computer, I have deleted all Microsoft preference files and trashed all of the Office applications, emptied trash, rebooted and reinstalled from
scratch. This did not work.
Using the Open command from Word did not work.
Not sure what else to try. I guess she gets to use Office 2011 (which opens these files just fine).
I do find it very frustrating that this issue has been around for so long and there have been no real fixes.