Preview highlights disappear

My highlights made in preview are disappearing. I constantly safe them and also restarted my mac in safe mode. I'm using Big Sur, which is tough the newest version I can install on my 2014 macbook.

I had this problem also years ago… not sure why it is happening again. Super annoying, as hours of work (comments, highlights) are gone. Also not all - just some of them- disappear, it feels super random and it's so frustrating to start again.

Any ideas?