Search engine doesn't show a result when searching by user key

The search engine at Developers (Account) section of the admin portal doesn't show a result when searching by user_key

Note from developer:

The solution in place allow search up to 125 keys of 256 chars length keys to be searched (or 124 because in the concat it has a space between each one of them). More if the size of the keys are shorter (which is usually the case).
The order of keys indexed is not guaranteed.

Why? This is a database compatibility restriction (we need to work with Oracle which limits to 32k characters each string)