Many book covers recently vanished

Hi there - I’ve been happily using the awesome micro.blog bookshelves feature for a long time now, making sure that every book I add has a cover selected (or I upload one if there isn’t one). Unfortunately recently I noticed that at least 30-40 of my books I added in the past are suddenly displaying a grey “book cover not available” icon instead of the cover when I know they all originally had covers! I tried rebuilding the site but it didn’t help. Is there a way to fix this problem? Many thanks.