Weird issue. I uploaded images to each of my products and until recently, the thumbnails appeared both on the back end and the front end of Products. However, lately, that has changed and now even though I uploaded the image, the thumbnail image does not appear even though I gave it the proper role of thumbnail. Why would that be occurring? I re-indexed the database, cleared cache, pretty much everything you would expect to do. This is Magento v2.2.5.