I was restoring a library (from iMac) on my Pi4 and noticed that, even though I backed up with the option to save Thumbnails on, none of the thumbnails actually were preserved. So I went to Tools>Advanced Tools>Recreate Thumbnails. That took a while and a few runs but eventually the Report stated that I had 100% thumbnails. However, in one of my views that shows all non-classical tracks, none of the thumbnails showed up (the view has a column for large thumbnails). The view that shows all classical tracks was fine.
The solution right now is to select all tracks, right-click and select Cover Art>Rebuild Thumbnails, which then successfully rebuilds the missing thumbnails. Also, this way, the rebuilding will run regardless of whether the thumbnails are there or not, whereas with b]Tools>Advanced Tools>Recreate Tumbnails[/b] there is no activity when all thumbnails are reported as built.
I was just wondering about this behavior and wish that the thumbnails would transfer successfully through a back-up/restore cycle.