Acrobat Reader: Advanced Search: untitled

The advanced search result shows for some files the name 'untitled'.  I could identify that this information is found in the Title filed of file description/metadata. I could change the information in the file and started the advanced search again. In search result the file name is again 'untitled'. 

I assume that there is  a cache. Is this correct? How can I delete the cache? is there an automatic process that deletes the cache? How often?

Is there an other reason why  search resullt don't show the correct title?

Win 10, Acrobat Reader latest version