So much for interoperability.
I'm doing some work in Powerpoint, taking an existing slide show and adapting a copy to make a Powerpoint for a different class that can re-use several of the same slides. The file I started out with was in the old (.ppt) file format, from Powerpoint 2003 and older. Somewhere along the way I saved my working copy in the newer .pptx (Powerpoint 2007 and newer) format, partially because it uses less disk space but mostly because I was getting messages about unsupported features. So I now have a fairly well-developed slide show in the new Powerpoint format.
Out of curiosity, I tried opening it up in "Presentations," which is the Powerpoint analog program in the SoftMaker FreeOffice suite from our friends in Germany. It opened okay. I found a couple of slides in which the line spacing in the main panel was a bit off, and I adjusted those. I didn't touch the header box on any of the slides. Then I saved it, closed it, and opened it again in Powerpoint 2016.
Oh, my!
What a mess. It completely bollixed the header boxes on EVERY slide. I've just spent over an hour repairing the damage.
At the moment I'm off to the supermarket. When I get back I'll open it again in FreeOffice, but this time I'll save it back as a copy, not overwrite the original. And now I'm curious to see what may happen if I open and save it in Libre Office. Should be interesting ...
Sheesh. SoftMaker advertises their suite as providing "flawless" compatibility with Microsoft Office. What I've been learning is that their word processor module isn't too bad on compatibility with Word, but their spreadsheet is less compatible with Excel. This is my first look at Powerpoint compatibility, and the result was less than satisfactory.