Why still thumbnails after clearing the cache?

I have a local vault on my IPhone. It was never connected to any cloud service. When I close the vault I always go to the settings of cryptomator and clear the cache. When I open the vault the next time I can still see thumbnail previews of my files. How is this possible? Shouldn´t the files be encrypted again after the vault was closed ?

Seems like iOS is creating caching thumbnails by them self and store them in their environment, independent from its source:

Is there a possibility to clear that cache? Where exactly is it?