WebM format

Also, we use Cornerstone LMS. They recommend MP4 and "WebM" is not listed at all. I'm I about to run into a playability issue with my LMS?

I'm not sure why that's happening, but as for playability, you're fine. See Can I Use.

I have seen the same. Every time I open any course, or reopen a course I recently closed, I get the message:

I can be working on the course for 2 hours, save it, come back later, reopen it, and get the message again. It seems it shouldn't need to be done every time you open the course.

I'm seeing that message also. Here is what is happening: https://www.articulatesupport.com/article/Storyline-360-WebM-Support