Hey HansC,
Welcome to Apple Support Communities.
From what we understand, you'd like to shuffle photos that falls under the categories "Nature" and "Cities", however all of the photos that would fall under those categories are not shuffling.
If you haven't already, try a different category. If the category doesn't display all photos that should be part of that category,
If the same issue occurs, make sure you iPhone is up-to-date. Keeping your iPhone up-to-date ensures security updates, new features, and fixes are installed on the device. See steps below if you need assistance with checking for updates/updating your iPhone.
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.
For future reference, the steps are also available in this article: Update your iPhone or iPad.
If you continue to notice a small percentage of photos shuffling, contact Apple Support here: Contact - Official Apple Support
Take care.