On Android 14 and later, photo access can be limited to only some of your photos. With this setting, only the photos you selected before are shown.
Open your device's "Settings" app → "Apps" → "Smart Planner" → "Permissions", tap "Photos and videos", and change it so that the app can access all of your photos. You will then be able to select all your photos.
※ You can also add more photos to the selection from the photo picker screen.