From my computer programming classes I know that when you change code base your first round goal is to try to keep everything the same and not lose or change features or functionality along the way. Word, Excel and PowerPoint had legacy code going back 30
years. This year is the 30th anniversary of Microsoft Office - it was Mac only back then.
If all the same old bugs are still the same in the new version, they actually did things right. Along the way, in Office 2016 preview a handful of bugs were fixed, but probably were fixed simply as a result of the new code being Cocoa based. I suspect nothing
in particular was done to fix the Word window position bug. It just happened to get fixed by switching code base. That's just a guess, though. I don't have inside info about it.
Now that the code base has been updated and the core features have been ported, the next logical step is to go on a bug-crushing binge. I presume that's why Microsoft made a public beta (the Preview), something they never did with Office for Mac before now.
There is a new feedback feature in the Office 2016 Preview. Click the smile button near the top-right corner of any app and a window opens that lets you include a current screen shot and even your email address with your submission.

You are correct that the resume feature is part of Mac OS X, not Microsoft Office.
I think as part of the public Preview, Microsoft is gathering feedback on feature and bug requests, as well. I would expect that after the core suite is decided to be stable, then they will get to work on addressing some of the finer points.
There are still major elements missing from the Preview edition. File > Properties isn't there. Toolbar support is missing. ODBC for Excel is missing. These will have to be finished first. Then they can move on.