Lightroom Classic 11.3.1 Release. If you check out the video you'll see I'm in Develop mode and clicking through the thumbnails results in other thumbnails going blank grey. Not an ideal feature. When I'm in Library mode thumbnails 9, 10, 14 and 15 are greyed out blank.
Things are obviously in a mess now as when I exit (with backup and optimize enabled) because LR won't exit and is stuck with this dialog. It will never finish and I will have to force-quit it.
After deleting it, LR will regenerate the previews on demand, which, depending on your photos and the speed of your computer, may cause it to be a little sluggish. You could go to All Photographs, select all photos, and do the menu command Library > Previews > Build Standard-Sized Previews and let it run over night.