I am also having the same problem for days. I thought I was going crazy. Finally yesterday I was able to confirm the issue and yes, after 2 hours of research, I was left with almost nothing. and now I have to redo it all over again. So frustrating.
The application is on the latest version, the same for Windows 11 I am using. I do use tasks and it is available in the UI. I only use the desktop version and the research is private so I am the only one that has access to it.
This is the first time that this message appears. Before the content would just be gone and left me with a lot of confusion and frustration.
I am honestly moving to One Note. I am not sure if it is any better but I can't trust that the report and research I do for work will still be there after the next sync.