iOS won't delete my app data. When I uninstall/reinstall the same data is always there...

This is an iOS 11 issue.  Basically, when you uninstall and app from your phone, if you don't delete the data for that app from iCloud, when you reinstall the app, it will "re-load" the previous data that was saved.  So as a result, you have to delete the data from iCloud  as well BEFORE re-installing the app.  The below article should help you clear your iCloud account of data for an app:

You have to remember that all your data from the downloaded apps is going to be stored on iCloud. Sometimes, this can lead to inconvenience for the user as there private information can be integrated with the data from apps. It can pose a security risk and take up a lot of space as far as overall storage is concerned. Even if you have deleted the apps that you do not need anymore, the data from those will still remain with iCloud.

In order to simply delete the app data from iCloud (iOS 11) to manage your iCloud storage, you can:

Step 1. On your iPhone, go to Settings and Press iCloud.
Step 2. Then tap on Storage and then Manage Storage.
Step 3. Under "BACKUPS", click on your iPhone name.
Step 4. Some of the apps will be listed there. Click "Show All Apps" to see all the apps.
Step 5. Go to the app that you want to delete data from iCloud, scroll it to the left.

Step 6. Confirm the action by tapping on "Turn Off & Delete".

This will turn off the backup of this app and also delete the app data from iCloud. It won't delete the apps from your iPhone.