PDF File thumbnails disappearing

Hello! For the last few versions or so, I’ve noticed that in Acrobat Reader for iPad, the PDF thumbnails occasionally disappear. Even if I close the app completely and then reopen it, the thumbnails don’t reappear (the classic red and black Adobe icon remains next to the file name). The only way to get the thumbnail back seems to be to delete the file and replace it with a previously saved copy. This problem started occurring when the small window showing some of the ‘Recent’ files was introduced in the ‘Files’ section, in the top-left corner of that section (previously, ‘Recent’ files were only available via the ‘Home’ section). I’m using a ninth-generation iPad and have installed the latest version of Acrobat Reader. I realise this isn’t a major problem, as the content of the PDFs remains unchanged, but it would be great if a solution could be found to prevent the thumbnails from disappearing. Thank you.