Empty "shared" date in the library

Why is the shared date empty in a lot of my shared lessons? I know the date was filled and when I unshare a lesson and share it again, the date is filled, but how does it happen that the date is so often empty?

Example:

I know that I unshared and shared the lessons to fill the date, but now there are empty shared dates again. One guess is that it happens if someone opens a lesson with the “editor” button.

@VeraI - Thanks for reporting this. We have noticed this one ourselves. I don’t know exactly what is causing it, but we do have it on our list and will be sure to take a proper look into it when we get a chance.