How do I delete leftover data from a game that has already been deleted from an iPad?

Yes. Downloading the (currently offloaded) App will re-associate the saved data with its parent App - after which you can properly delete (not offload) the App. App deletion removes both the App and all associated data.

iOS/iPadOS currently offer no other easy method to delete App-data that is associated with an offloaded App. The only other method is to delete all locally stored data from iPad settings.

It is generally recommended that you maintain 4-6GB of free space at all times - as your iPad needs some temporary space to complete some tasks. If your internal storage is full, or is insufficient for the task that you need to perform, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

I'd contact Genshin Impact developer/support team and ask them.

Use these instructions:--> How to contact an app developer - Apple Support

Regards

Giulio

The 26 GB of the leftover app is app data remaining after you offloaded the app. Make sure to delete it.

Make sure that you deleted the app instead of offloading it.