Do you have "Optimize iPhone Storage" enabled in the settings for iCloud Photos?
If yes, all your photos will be shown in a low resolution on your device, the full resolution photos are stored in iCloud Photos and can be downloaded by tapping on the photos:
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When Optimize Storage is turned on, full-resolution photos and videos are stored in iCloud, and, when needed, space-saving copies are stored on your device.
Si Optimizar almacenamiento está activado, las fotos y los vídeos con resolución completa se almacenan en iCloud y, cuando es necesario, se almacenan copias que ocupan menos espacio en el dispositivo.
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copied from: Manage your photo and video storage - Apple Support
Administrar el almacenamiento de fotos y vídeos - Soporte técnico de Apple (ES)