It happens to the photos in the sent items. because at this time the gallery app doesn't read or doesn't open the sub folder inside the WhatsApp Images folder.
The way to open the photos in the sent WhatsApp images by opening them using the File Browser App. (not using the Gallery App).
Browse to the WhatsApp sent items folder:
Storage -> Internal storage (or External storege if you already change the default storage) -> WhatsApp -> Media -> WhatsApp Images -> Sent.
