Hello!
We're grateful for your participation in the Apple Support Community. This can be a frustrating issue. Here are some steps that you can try to solve it:
Sometimes, the problem can be related to iCloud. You can try signing out of your iCloud account and then sign back in. If your photos are synced with iCloud, you can try to clear the iCloud Photos cache on your device. This is done by going to Settings > [Your Name] > iCloud > Photos and disabling iCloud Photos. Restart your device and then re-enable iCloud Photos. If the problem persists, you can try to reset all settings on your iPhone. Please note that this will reset all your settings to their defaults, but your data and apps will remain untouched. You can do this by going to Settings > General > Reset > Reset All Settings.
Make sure your device is running the latest version of iOS. Sometimes, updating the system can fix this type of issues. If none of the above solutions work, I recommend reaching out to Apple Support Tel.: 0800 6645 451 (DE) for further assistance. Remember to back up your device before attempting these steps, especially before resetting all settings, just to be on the safe side.
For those who prefer English, we have an exclusive community available at discussions.apple.com.
As your post is in English, we cordially encourage you to join our English-speaking community there.
Thank you!