Have you tried running the scheduled job for clearing thumbnail properties? I would run that job to see if that heps you., Essentially what is happening behind the scene is episerver is holding on to the thumbnail url for the content item but the file doesn't exists. Clearning the thumbnail will remove the url from that property forcing it to be recreated the next time it is called and notices it doesn't have a url in the thumbnail property.
I used Geta/Epi.Font.Thumbnail add-on(Geta Thumbnail) for generating thumbnail icon in Episerver CMS V11.
The thumbnail got generated but I deleted it the first time.
Now thumbnail is not getting generated in App_Data/Thumb_Cache Folder.
can anyone assist?