Not new and I've experienced this ever since I started using DS.... all the versions. I don't have to restart because I can fix it easily as mentioned above. It's just annoying. I'm now in the habit of not saving unless the Content Pane is open, if I remember to.
I looked on the Daz forum and I'm not the only one - "I need to minimize, maximize Daz Studio, click another tab, then click "content library" for the icons to reappear."
I did a test and found that if I save to C drive there's no problem, but when saving the same scene to SSD it reappeared. So it's definitely SSD related. Not a big problem so I'll just have to live with it.