I have a new install of Photoshop Elements 2021 on a new Win10 PC.
After importing *.jpg files from a Samsung phone, only some of the photos are shown in the Organizer view, even if all photos are stored in the folder.
However, when using the Photo Editor, all photos are shown correctly in the File Open dialogue. I opened one of the "hidden files" in Photo Editor and saved it with a different name, this photo then displays in Organizer.
I have checked that View/Hidden Files/ is set to Show All Files.